FS 209-Anatomy and Biology of Shellfish

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FS 209-Anatomy and Biology of Shellfish 2(1+1)

(ICAR e-course material)

PART I-Anatomy

ARTHROPODA

12.1. General characters of Arthropoda

·         Arthropods are triploblastic, bilaterally symmetrical, metabolically segmented animals.

·         Body is covered with a thick chitinous cuticle forming an exoskeleton.

·         Body segments usually bear paired lateral and jointed appendages.

·         Musculature is not continuous but comprises separate 'striped muscles.

·         Body cavity is haemocoel. The true coelom is reduced to the spaces of the genital and excretory organs.

·         Digestive tract is complete; mouth and anus lie at opposite ends of the body.

·         Circulatory system is open with dorsal heart and arteries but without capillaries.

·         Respiration through general body surface, by gills in aquatic forms, tracheae or book lungs in terrestrial forms.

·         True nephritic are absent. Excretion by coelomoducts or Malpighian tubules or green or coxal lands.

·         Cilia are entirely absent from all parts of the body.

·         Sexes are generally separate and sexual dimorphism is often exhibited by several forms.

·         Fertilization is internal. Development is usually indirect through larval stages.

·         Parental cave is also often well marked in many arthropods.

Shrimps

12.1.1.Shrimps - External Anatomy
External Anatomy
As a decapods crustacean, the white shrimp, Penaeus (Litopenacus) sctiferus, is rather primitive. It has been selected for use here because it illustrates not only the structure of a shrimp, but also the generalized body plan of a decapod crustacean.

A large portion of the white shrimp, as of any other shrimp, consists of muscle and shell, or exoskeleton. In fact, the largest of the three natural divisions of the body; namely, the abdomen or "tail," consists of little other than muscle and shell. Two main masses of muscle are the "meat" of the shrimp's tail: (1) the relatively small dorsal abdominal muscles, which lie above the intestinal tract, or gut, and above the dorsal abdominal artery, both of which are removed in preparation for eating, and (2) the large ventral abdominal muscles, which extend from either side of the intestinal tract and dorsal abdominal artery ventrally to both sides of the ventral abdominal nerve cord.

For swimming quietly, the white shrimp uses its five pairs of abdominal appendages, known as pleopods. But when the shrimp moves rapidly, it does so by contracting its ventral abdominal muscles and curving forward its tail fan, which is composed of a centrally situated telson and a pair of lateral appendages known as uropods. The powerful thrust exerted by tail and tail fan upon the water propels the shrimp backward with extraordinary speed. The tail of the shrimp returns to its normal, more or less elongated, position by the contraction of the dorsal abdominal muscles, which act as extensors. The tail's flexibility results from deep folds of thin, soft chitin that link the six segments of the tail to one another.
Within the cephalothorax of the white shrimp are large portions of the digestive, circulatory, nervous, and reproductive systems. The long digestive tract, or gut, has three main subdivisions known, respectively, as foregut, midgut and hindgut. Food particles picked up by the mouth parts are ground by the mandibles and swallowed, whereupon they enter the narrow, tubular, muscular esophagus, which is the initial portion of the foregut. Lined with chitin, the esophagus nonetheless can accommodate large amounts of food since it has one anterior and two lateral folds loosely filled with connective tissue. When these folds become unfolded, the esophagus can distend greatly.

12.1.2. Digestive system
From the esophagus, food particles enter the anterior chamber, the second portion of the chitin-lined foregut. Many authors have called this the cardiac stomach. The anterior chamber has lateral longitudinal folds that permit it to expand when filling with food. The anterior chamber also has ventrally situated longitudinal ridges that lead back to the openings of the midgut glands in the caudad part of the posterior chamber, frequently called the pyloric stomach.
Walls of the anterior chamber contain a triangular structure consisting of a median tooth and a row of tooth-like denticles along each side. When food enters the anterior chamber, the muscles that insert on the chamber alternately contract and relax, thereby causing the median tooth to move against the denticles and lateral ridges. In so doing, this grinding apparatus, termed the gastric mill, breaks down the food into very fine particles.
While food is within the anterior chamber, it is mixed with digestive juices that flow forward ventrally from the posterior chamber. The juices enter the caudad part of the posterior chamber viaducts that originate in the lumen, or cavity, of many-branched tubules constituting the paired midgut glands. Thus, the lumen of the tubules is continuous with the lumen of the gut.
Digestion of food takes place partly in the anterior chamber, partly in the posterior chamber, and partly in the tubules of the midgut glands. In the posterior chamber, there is a filter formed by two lateral ridges and one ventral median ridge densely covered with hair-like setae. Owing to this filter, only fluid and minutely divided food particles can pass from the posterior chamber into ducts leading to the midgut glands and thence into its branching tubules for further digestion. From the midgut glands, end products of digestion are readily absorbed into the hemolymph.
Fine indigestible material within the midgut glands is forced back into the posterior chamber and then into the straight, unlined, tubular portion of the midgut. Here end- products of digestion enter the hemolymph via the many small blood vessels connecting the tubular portion of the midgut with the dorsal abdominal artery just above. Here also the fine indigestible material is mixed with larger indigestible particles that had been filtered away from the openings of the midgut glands and had passed directly from the posterior chamber into the tubular part of the midgut.
Within the midgut, indigestible material is packaged into long fecal pellets and enclosed within a membrane, the pentrophic membrane (from the Greek, pen, around; troplio, feed), which is secreted by epithelial cells of the midgut and is mucoid in nature. Strong peristaltic contractions of the mid- gut push-the fecal pellets along to the chitin-lined hindgut, which is enlarged as a rectum. A series of rapid contractions by the rectum then forces the fecal pellets out of the body by way of the anus.
At the junction of midgut and hindgut in the sixth abdominal segment, the midgut gives rise to a diverticulum, called the posterior midgut cecum by some authors and the hindgut or rectal gland by others. The function of this organ is not known, but its cells appear to be secretory.
Presumably the midgut would be distinguishable from the foregut and hindgut by its lack of chitinous lining. Thus, the esophagus, anterior chamber, and cephalad portion of the posterior chamber are lined with chitin and are clearly foregut. The caudad portion of the posterior chamber also is lined, although incompletely, with chitin; yet this is midgut. Dorsally and laterally, the chitinous lining of the foregut in the white shrimp (and in many other decapod crustaceans) extends into the midgut well past the openings Of the midgut glands; ventrally a caudad extension of the chitinous lining separates the openings of the midgut glands and also covers the epithelium of more posterior portions of the midgut for some distance. These caudad extensions of the chitinous lining probably direct sand and other indigestible particles to the peritrophic membrane for packaging without damage, en route, to the delicate area around the openings of the midgut glands.

12.1.3. Circulatory System
The heart of the white shrimp has three pairs of small openings known as ostia. Through these ostia, the blood flows into the heart from the surrounding area, which is termed the pericardial sinus, or pericardium. Valves prevent the blood from leaking out through the ostia as the heart contracts. Instead, the blood is driven into major arteries, most of which run forward to supply blood to the sense. organs and to vital organs within the cephalothorax. However, the sternal artery runs to war ventral region of the shrimp, where—it gives rise to a ventral thoracic artery that supplies blood to the thoracic appendages and to the thoracic portion of the ventral nerve cord. The dorsal abdominal artery leaves the heart posteriorly and supplies blood to the gut, the abdominal muscles, and the abdominal portion of the ventral nerve cord.

12.1.4. Nervous System
The nervous system of the white shrimp consists of a brain (supraesophageal ganglion), which is situated dorsally in the head, two circumesophageal connectives that pass on either side of the esophagus and are connected with each other by the tritocerebral commissure, and a ventral nerve cord, which runs posteriorly the entire length of the shrimp and at more or less regular intervals is swollen into bulbous ganglia. The entire central nervous system of the white shrimp, as of other decapod crustaceans, is fundamentally "ladder-type" in structure, but in most regions the two longitudinal halves of the "ladder" have fused. As a consequence, the word "ganglion" generally refers to a pair of laterally fused ganglia.
The brain receives nerves from sense organs of the head, notably the eyes and antennae, and supplies nerves to the muscles that operate these sense organs. In the ventral nerve cord, the first ganglion (subesophageal ganglion) and the remaining ventral ganglia (five in the thorax and six in the abdomen) receive nerve fibers from sensory cells widely dispersed through the body of the shrimp and supply nerves to muscles that move the mouth parts, thoracic legs, pleopods, and tail.
In addition, lying on the circurnesophageal connectives is a pair of connective ganglia, or stomatogastric ganglia (stö-mto-GAS-trik; from the Greek, stoma, mouth; gaster, stomach). The connective ganglia and the stomadeal ganglion on the anterior surface of the çsophagus combine to form the stomadeal system, which supplies nerves to the esophagus and the foregut.
In the forward part of the cephalothorax of the white shrimp, situated on the second, or antennal, segment are the kidneys, which because of their location are often called antennal glands. Each kidney is made up of a small dorsal portion that lies above the brain and a large ventral portion lying beneath the brain. The two portions of each kidney are connected with each other by lateral arms. Part of the ventral portion extends into the antenna on the same side of the animal. A short duct from this portion of each kidney leads to the exterior through an excretory pore, which lies at the base of the antenna on its inner (medial) side. In higher shrimps, the Caridea, a bladder also is present.
There are 19 pairs of gills in Penaeus setiferus. Three pairs occur in each thoracic segment, except the first and last, where there is one pair. In any given segment the gills may be attached to the base of the limb, to the flexible membrane between limb and body, or to the body wall. A gill of the white shrimp consists of a primary supporting axis known as a rachis (RAY-kis; from the Greek, rhakhis, spine, or backbone), from which secondary supporting structures emerge at right angles. On the secondary supporting structures are many gill filaments that in turn protrude at right angles. Each secondary supporting structure with its attached filaments nests against the preceding one. In caridean shrimps, the gill filaments are flattened and plate-like and protrude directly from the primary supporting structure; this type of gill also is found in crabs.

12.1.5. Respiratory System
In shrimps, as in all other decapod crustaceans, the gills lie within two branchial chambers, each of which results from a deep lateral fold of the carapace. The beating of a leaf-like flap, the gill bailer, or scaphognathite, causes water to enter the branchial chamber from below and behind that is, through opening between the thoracic legs and in front of the abdomen. The water leaves the -branchial chamber through 'a channel, directed toward the head, in which lies the beating gill bailer. As the water circulates through the branchial chamber, an exchange of gases takes place between the water and the blood in the gill filaments. At the same time there is a discharge of excess salts from the blood into the water and an uptake of needed salts from the water into the blood.

12.1.6. Reproductive System
In the white shrimp the most conspicuous components of the female reproductive system are two ovaries that extend, partially fused, from the anterior of the foregut posteriorly to the tail fan. The portion of each ovary that is within the cephalothorax consists of a forward-projecting lobe, which lies close to the esophagus and chambers of the foregut, and seven finger-like lateral lobes, which are situated above the midgut gland and beneath the heart. This arrange-ment makes the heart resemble a saddle straddling the ovaries. The abdominal portion of the ovaries consists of two lobes, lying above and to the sides of the intestine and below and to the sides of the dorsal abdominal artery.

Emerging from each ovary at the sixth lateral lobe is an oviduct. Coursing ventrally, each oviduct opens to the exterior a genital pore situated medially on the basal segment of the third thoracic leg. The opening is concealed within an ear-shaped protuberance covered with setae.
Externally and posterior to the genital openings of the female lies a structure that is adapted for receiving a packet of sperms, or spermatophore (sper-MA-t5-phôre; from the Greek, sperma, seed; phoros, bearing) from the male during mating. Known as thelycum (THEL--cüm; from the Greek, thelys, female), this structure consists of several lobes and protuberances bearing stiff bristles.
The male reproductive system of the white shrimp includes a pair of partially fused testes that lie in a position quite similar to that of the ovaries in the female. Each testis has an anterior lobe projecting forward over the chambers of the foregut and six lateral lobes that lie over the midgut gland and under the heart. In place of a long abdominal lobe as in an ovary, each testis has a short posterior lobe.
A pair of ducts known as the vasa deferentia emerge from the main axis of the testes at their posterior margin, course ventrally, and open to the exterior at the genital pores situated medially on the basal segment of the fifth pair of thoracic legs. Each vas deferens has four distinct regions: a short, narrow proximal portion; a thickened, doubly flexed medial portion; a long, narrow tubular portion; and a much dilated, muscular terminal ampoule. Within the terminal ampoule the spermatophore is formed.

The spermatophore of the white shrimp roughly resembles a pod. It consists of two halves, each of which contains sperms enclosed within a sheath and surrounded by chitin. The thoracic legs of the male shrimp presumably assemble the spermatophore immediately after each half is expelled from the terminal ampoule of the corresponding vas deferens. The legs place the spermatophore within the trough of the petasma, a structure that results from modification of the first pair of pleopods. The petasma consists of stiffened longitudinal rods and folds of soft chitin that, when unfolded, result in a broadly inflated male copulatory organ.

During mating, the male shrimp uses the petasma to thrust the spermatophore against the thelycum of the female. Here bristles on protuberances of the thelycum overlap the spermatophore, thus helping to secure it. Two lobes known as "wings" on the spermatophore become anchored in a groove on the ventral surface of the female between her third and fourth thoracic legs. Despite these devices for securing the spermatophore, it is easily dislodged, and spermatophore bearing females of white shrimp are not commonly caught in shrimp trawls. A pair of light-colored, pad-like structures situated just posterior to the thelycum are believed to play no role during impregnation.

Lobsters

12.2.1.External Anatomy
In general body plan a lobster does not differ greatly from a shrimp. A lobster has the same type of muscular abdomen, or tail, which undergoes sudden flexion by contraction of the large ventral abdominal muscles and more leisurely extension by contraction of the smaller dorsal abdominal muscles. As in shrimp, the tail of a lobster provides the animal with its surest means of escape- jetlike propulsion backward.

In lobster as in shrimp, a carapace covers the head and the thorax and, except for the presence of the cervical groove, obscures the boundary between these two regions. In lobsters, the cephalothorax is commonly called the "body," while in shrimp this same region is known as the "head."
Despite similarities in body plan, it is quite easy to distinguish a lobster from a shrimp. Even as an adult, a shrimp is relatively small and its shell is somewhat fragile. An adult lobster, on the other hand, may reach very large size and acquire an extremely hard shell. Furthermore, a lobster is compressed dorsoventrally (from top to bottom), not laterally (from side to side), as is a shrimp. True lobsters have yet another distinguishing characteristic: their first pair, of thoracic legs is modified as large claws, or chelipeds. In some species, such as the American lobster, Homarus americanus, one large claw, the crusher, is much heavier than the other claw, known as the pincer, or the biting, cutter, or ripper claw. The crusher of the American lobster occurs about as frequently on the right side of the body as on the left. These are large & claws are lacking in the spiny or rock, lobsters.

As in a shrimp, many vital organs of a lobster are situated under the carapace within the cephalothorax. Here are the chitin-lined foregut, at least a portion of the midgut, and the midgut glands. Here also lie the brain, heart, gills, excretory organs, and a large part of the male and female reproductive organs.

12.2.2. Digestive System
In both true lobsters and spiny lobsters, the gastric mill is more highly developed than in the white shrimp. The gastric mill of the lobster is largely restricted to the region of the foregut in which the large, thin-walled anterior chamber gives way to-.the much smaller, thick-walled posterior chamber. At the constriction between the two chambers, three movable teeth, one median and two lateral, are attached to small, hard skeletal plates known as ossicles. These teeth chew the food, which arrives in the anterior chamber as long, stretched, but unchewed pieces. A well-developed gastric mill is a useful device enabling a decapod crustacean, when safely hidden from its enemies, to chew s food at leisure, after having swallowed it in large pieces. The gastric mill is least developed in such decapod crustaceans as the shrimps, in which the mouthparts chew the food quite thoroughly before the food enters the esophagus.
In the walls of the anterior chamber, a lobster has many ossicles in addition to those of the gastric mill. These additional ossicles serve as a place of attachment for muscles that move the foregut and thereby enable the ossicles of the gastric mill to grind the food. Once the food has been ground thoroughly, it passes through a setose filter that prevents all but the finest particles from entering the mid gut glands through ducts that open into the unlined caudad portion of the posterior chamber.
The midgut of the American lobster is long, extending back to the last abdominal segment, where it connects with the chum-lined hindgut, which has become modified as an enlarged rectum. A posterior midgut diverticulum, or cecum, arises just in front of the junction of midgut and rectum. Undigested wastes are egested from the rectum through the anus. In spiny lobsters, the midgut is very short, while the hindgut is long and contains many longitudinal folds. No enlarged rectum is present, the terminal portion of the hind- gut being narrow and very muscular. By their contraction the muscles of the hindgut force undigested (fecal) material out through the anus.

In the foregut, midgut, and midgut glands of the American lobster, digestion of food takes place through the action of digestive enzymes that are secreted by the midgut glands. These glands are also the principal site for absorption of digested food and for storage of reserve food materials. Chefs call the midgut glands of the lobster the tomally; accumulated food reserves make the tomally rich and flavorful when cooked. The tomally can easily be recognized, for it is soft, large, and many-lobed, and, in color, i-1 green, bright yellow, yellow-green, or yellow-brown.

12.2.3.Circulatory System
The circulatory system of a lobster is not very different from that of the white shrimp. In lobsters as in shrimps, the heart lies under the middorsal surface of the cephalothorax just in front of its junction with the abdomen. Three pairs of ostia allow blood that has collected within the pericardium to flow into the heart when the organ relaxes. During contraction of the heart, the ostia close and prevent the blood from flowing back into the pericardium.
From the heart;-the blood flows forward through several arteries to vital organs within the cephalothorax. The blood also flows into the dorsal abdominal artery and its paired branches in each segment. These supply blood to the ventrally situated flexor muscles and the dosal1y situated extensor muscles of the abdomen. A sternal artery carries blood to the gonads, then courses ventrally to give rise to the ventral thoracic artery and the ventral abdominal artery. In lobsters, as in shrimps, the ventral thoracic artery carries blood to most thoracic appendages and to the thoracic portion of the ventral nerve cord. In lobsters, but not in shrimps, the ventral abdominal artery extends through the abdomen, supplying blood to the last two pairs of thoracic legs, the ventral nerve cord, the posterior part of the hind gut, and the tail fan.

12.2.4. Nervous System
The central nervous system of the American lobster differs little from that of the white shrimp. Lobsters, like shrimps, have a brain, or supraesophageal ganglion, composed of several fused paired ganglia. Running ventrally and posteriorly from the brain are two circumesophageal connectives, a slight swelling on each connective as it passes the esophagus marking the position of the stomatogastric, or connective, ganglia. Behind the esophagus, the connectives are joined by the small tritocerebral commissure.
Due to fusion of the first two thoracic ganglia with three cephalic ganglia to form the subesophageal ganglion, the thoracic portion of the ventral nerve cord in the American lobster contains only five additional ganglia (ganglia of the last two thoracic segments have fused with each other). The abdominal portion of the Ventral nerve cord contains six ganglia, one in each segment. This arrangement of thoracic and abdominal ganglia is similar to that in the white shrimp. In spiny lobsters, the thoracic ganglia have undergone greater fusion. Dr. C. J. George and his co-workers at Wilson College, Bombay, India, reported that in the thorax of Panulirus polypizagus there are only two ganglionic masses. The larger, anterior ganglionic mass has resulted apparently from fusion of nine pairs of ganglia (three cephalic, six thoracic), while the smaller, posterior ganglionic mass has come from fusion of two thoracic pairs. Yet in its abdomen, Panulirus polyphagus retains the original number of six ganglia. As in shrimps, the brain of lobsters receives nerves from sense organs of the head, notably the eyes and antennae. Ganglia of the thorax supply nerves to the mouth parts and thoracic legs. Abdominal ganglia furnish the nerve supply to flexor and extensor muscles of the abdomen, to the intestine, and to the abdominal appendages.

12.2.5. Excretory System
The kidneys of lobsters, like those of shrimps, are known as antennal glands. More compact than in shrimps, the kidneys of lobsters have a pale olive-green hue and thus are often called green glands. They lie on each side of the body, below and in front of the foregut. Urine that is formed in a glandular portion passes into tubes that enter a duct leading from a dorsally situated bladder. There is no direct connection between bladder and glandular portion, so the bladder can be filled only when urine backs up through this duct. Subsequently, the urine is released to the exterior via the same duct, which opens on the basal segment of the antenna.

12.2.6. Respiratory System
Gills of lobsters are of a type known as trichobranch (TRIK-o-brank; from the Greek, thrix, hair; branchia, gills), for they are composed of numerous filaments arranged, plume- like, around a central axis. As in shrimps, on any given thoracic segment there may he as many as four pairs of gills, one pair on the basal segment of the limbs, two pairs arising from the soft membrane linking the limbs to the body, and one pair on the side of the body just above the limbs. In the American lobster the full complement of gills occurs at the base of the second, third, and fourth thoracic legs, with fewer pairs on the remaining thoracic segments except the first, which lacks gills. In all, there are 20 pairs of gills in the American lobster. On each side the gills lie within the branchial chamber, which is formed, as in shrimps, by a deep lateral fold of the carapace. Access to the brachial chamber is through very small openings between the appendages and two larger Openings, both ventral, one at the posterior end of the branchial chamber and the other at its anterior end. In a channel at the anterior opening is the leaf-like flap known as the gill bailer, or scaphognathite, which by its rapid beating drives water forward in the channel and out of the branchial chamber. At the same time the current thus established within the branchial chamber causes water to enter the ventral and posterior openings, principally the latter. Every few minutes, the gill bailer reverses is beat for a few strokes, thereby causing the current of water to flow in the opposite direction. By this reversal of current, silt and other debris that may have settled on the gills are loosened and can be flushed from the chamber.

12.2.7. Reproductive System
In the American lobster the ovaries of the female appear in the form of a letter H, with the cross bar at the forward margin of the heart and with longitudinal lobes extending forward and backward through much of the animal. The stage of ovarian development is apparent from the color, bright yellow or flesh-colored early in development, then salmon, light green, and finally a rich dark green by maturity. After cooking, the mature, egg-filled ovaries are bright red and are known as the coral.
From the ovaries, paired ribbon-like oviducts emerge at a level just below the heart, then quickly narrow as they run outward to the body wall and downward to the base of the third pair of thoracic legs, where they terminate on the inner surface of the basal segment. Externally and medial these openings is a triangular, bluish structure extending from the base of the third to just beyond the base of the fourth pair of thoracic legs. This is the seminal receptacle, a small pocket in the exoskeleton that receives sperms from the male during mating.

In males of the American lobster the testes, which are pale tan-grey in color, may be H-shaped, like the ovaries of the female, or longitudinally paired, without a cross bar. From the testes, paired ducts, the vasa deferentia, emerge beneath the heart, at approximately the same place that the oviducts emerge from the ovaries. Like the oviducts, the vasa deferentia run outward to the body wall before turning downward. At this point they become S-shaped, with their posterior margin thickened and glandular, capable of secreting a gelatinous material that coats the sperms as they pass through the duct. The vasa deferentia then become briefly bulbous and muscular and, following this, narrow and thin-walled, forming an ejaculatory duct that opens at a papilla on the inner surface of the base of the fifth (and last) pair of thoracic legs.

The most obvious external difference between male and female American lobsters lies in the shape of the first pair of abdominal appendages. In the male these are the copulatory pleopods, relatively long, hard, grooved, and tapering. In the female these pleopods are small and soft. Yet there are other sexual differences, for mature males are heavier and have lager claws and a longer, more swollen carapace than have mature females.
Also in spiny lobsters, the sexes can he separated by differences in the abdominal appendages. In males the pleopods have one leaf-like terminal segment. In females the pleopods have two terminal branches, those of the first pleopods being leaf-like, while those of more posterior pleopods have one ref1ike branch and one rod-like branch used for attachment of eggs. In addition, the fifth pair of thoracic legs in male spiny lobsters terminates in a single, simple segment like that of more anterior legs, whereas in females the fifth pair of thoracic legs terminates in a small claw used in cleaning the attached eggs. Mature males tend to be larger than mature females.
Before leaving the subject of structure in American lobster, we may give some thought to coloration, for it can be surprisingly variable and frequently serves a protective function, enabling a lobster to blend with its background.
Normal, or "wild-type," coloration of American lobsters is mottled olive-green or dark blue-green above, with small black or-green-black spots and often red tubercles and spines. On some lobsters the sides of the body and tail, as well as large portions of the claws, may be dusky orange, often dotted with green-black. Other lobsters are almost entirely dusky orange, with green-black spots. Such variations in color exist among lobsters of widely differing sizes, from the one-pound individuals commonly purchased in fish markets to the lobsters of 10 to 15 pounds or more that are caught on the southeastern part of George's Bank and in areas to the south.
American lobsters may be of other colors as well. Some, known as calico, or leopard, lobsters, are light yellow with purple-blue marbling or spots. Other lobsters are rich indigo blue, with bright, clear blue on the sides of the body and on the extremities. Sometimes lobsters are pale red, hardly distinguishable from the cooked animal when seen from above. Yet, whatever their color topside, live American lobsters tend to be very lightly pigmented, or even cream-colored, underneath.
Occasionally, fishermen catch American lobsters that are cream-colored above as well as below, but with dark eyes and often with red pigment on the underside of the claws. Or such a cream-colored lobster may have faint traces of blue in its shell, as did one that was exhibited in Boston at the New England Aquarium. Such lobsters are frequently called albinos, although true albinos lack all pigment in eyes and shell. True albino American lobsters apparently have never been taken.
No single factor is responsible for the differences in color of American lobsters. The basic color pattern is inherited, just as are color and texture of hair in man and other mammals. But in an American lobster the actual color that develops may depend partly upon the type and strength of illumination to which the animal is exposed and even more upon its diet.
Thus, Professor F. H. Herrick, who in 1895 published .a classic monograph on the American lobster, observed that bluish coloration in this animal can result from prolonged exposure to sunlight. Recently, John T. Hughes and George C. Matthiessen of the Massachusetts Division of Marine Fisheries reported that lobsters held for a period of years at the Division's lobster hatchery and rearing facility in Oak Bluffs, Martha's Vineyard, and fed primarily quahaugs, clams, scallop viscera, and alewives, turned a deep sky-blue color, which eventually faded into a pale blue—grey. When, however, these bluish lobsters were then fed exclusively on green crabs, they reverted somewhat to the wild-type coloration after the next molt and became identical in coloration with the wild-type following the second molt.

Color in all decapod crustaceans results primarily from the presence of pigments known as carotenoids (after carrots, from which they were first isolated) in the tissues and shell. The major carotenoid of decapod crustaceans is astaxanthin, which is bright red in color. When combined, or conjugated, with protein, the red color of free astaxanthin is replaced by a color characteristic of the particular conjugated protein that is present. For example, in the shell of American lobsters, the most abundant pigment usually is a conjugated protein of astaxanthin that is blue. Eggs of American lobsters contain a green conjugated protein. Green crabs about to molt have a green conjugated protein in the old shell and a brown one in the epidermis and pigmented layers of the new shell.
The reason that diet plays such an important role in development of color in American lobsters and other decapod crustaceans is that carotenoids present in the conjugated proteins of these animals have to be either ingested or produced in the animal's body from ingested carotenoids. These pig-ments cannot be synthesized from noncarotenoid material, except by plants.
Shrimps, lobsters and crabs turn red when they are cooked because heat breaks down the linkage between astaxanthin and protein, and the astaxanthin is freed. Shrimps, lobsters and crabs that are red before being cooked do not have free astaxanthin, but rather an astaxanthin—protein complex that is red in color.
With regard to coloration, there is an important difference between shrimps, on the one hand, and lobsters, crayfishes and crabs, on the other. This concerns the way in which the colors are manifest. Shrimps have a light, fragile, quite transparent shell, through which the underlying integument is visible. In the integument are numerous pigment-containing cells known as chromatophores. Under the influence of certain hormones that originate within the central nervous system and are released into the hemolymph, the pigments within the chromatophores either concentrate in the center of the cell or migrate to the periphery, as the case may be.
Chromatophores have many branches, and thus a cell in which the pigments are dispersed looks very different from one in which the pigments are assembled into a tiny mass at the center. Furthermore, the area covered by chromatophoral pigments when they are dispersed is much greater than when they are concentrated, so the degree of pigment dispersion largely determines the overall coloration of a shrimp. This may change, rapidly and frequently, in response to changes in illumination and color of background, a fact that explains why common names for shrimps often include some that are descriptive of very different colors.
In lobsters, crayfishes and most crabs, the shell is thick, strong, and largely opaque, due to pigments that are deposited within the shell. Hence, in these decapod crustaceans, the color of the animal is fairly constant, depending primarily upon the color of pigments within the shell rather than upon the degree of dispersion of pigments within the chromatophores. Only in. certain restricted area is the shell of a lobster, crayfish, or crab more or less transparent, and here the color of the underlying pigments can be seen. In a few crabs, notably the fiddler crab Uca pugilator and the ghost, or sand, crab Ocypode, the shell is fairly light and semitransparent, and overall coloration results largely from pigments within the chromatophores.
Sometimes the left half of an American lobster (or of its close relative, the European lobster, Homnrus gammarus) may be of one color and the right half quite a different color. Professor Herrick and several later investigators described a number of such particolored lobsters: light yellow/bright red; dark green/pale red; blue/white; green-black/light orange; dark green/sky blue; dark blue/light red; dark green/red; white-red/purple.-blue.
In one case a bilateral difference in color of American lobster was correlated with a bilateral difference in sex. In 1959, Dr. Fenner A. Chace, Jr., and Dr. George M. Moore described an American lobster that on its left side was orange, with mottling and spots of dark green-brown and on its right side was similarly mottled and spotted but mostly in shades of blue over a light, blue ground color. Externally, the lobster appeared female on the right side and male on the left side. When the lobster was dissected, it was found to have well developed female reproductive organs on the right side and male reproductive organs on the left. In three earlier reports by other scientists, American lobsters having both male and female reproductive organs were described, but in no case was the bilateral difference in sex associated with a bilateral difference in coloration.

Crab

12.3.1.External Anatomy
We have seen that although crabs appear to be tailless, they have a very small tail, which they keep tucked underneath their body. Due to its small size, this tail and its appendages cannot be used for locomotion. The thoracic legs of a crab are used for walking. In certain crabs, including the blue crab, the last pair of thoracic legs is flattened and paddle-shaped and is used for swimming.

While the tail of shrimp or lobster is among the meatiest and most succulent portions of the animal, the tail of a crab contains little meat. The dorsal abdominal muscles are small and very weak, being used solely to extend the tail backward. Virtually the only time at which these muscles are used is during mating, when the abdomen of both male and female must be drawn backward to permit the transfer of sperms.

The ventral abdominal muscles of crabs are somewhat heavier and stronger, particularly in the mature female. While carrying eggs, she used these muscles to curl her broad, rounded abdomen over the mass of eggs. When not carrying eggs, she uses these same muscles to hold her abdomen tightly in a depression on the ventral surface of her body. Male crabs have a “locking device” consisting of small tubercles on the fifth thoracic segment that secure the triangular or T- Shaped abdomen in a depression on the ventral side of the thorax Covering both head and thorax of a crab dorsally is a hard carapace. Thus, the boundary between the two body regions is obscure and, as in shrimps and lobsters, one generally speaks of a cephalothorax rather than of the two separate regions. The cervical groove, indicating the boundary between head and thorax, lies just behind the center of the carapace, where it runs generally forward and to each side.

Ventrally, the boundary between head and thorax is well marked, as is the division of the thorax in to segments, although only the last five may readily be visible. Also the attachment of the thoracic legs to the exoskeleton is clearly apparent, one pair on each of these last five thoracic segments. The first pair is modified as chelipeds, or claws, while the remaining four pairs are adapted for 'balking or, in some cases, for walking and the last pair for swimming. In two families of primitive crabs (Dromiidae, Dorippidae), the last pair or last two pairs of thoracic legs ar held dorsally, often supporting a piece of sponge or bivalve shell or some other type of sheltering material.
The cephalothorax of a crab is characteristically short and broad and, in some species, greatly extended to the sides. In the blue crab, Callinectes sapidus, the paired, widely expanded branchial regions of the carapace terminate in a long, sharp lateral spine. Here the exoskeleton turns sharply inward and downward, to end just above Ube legs. As a result of anterior—posterior compression and lateral expansion, the branchial chambers are short and wide. Within these chambers the gills are, of necessity, arranged in a broad oval, rather than linearly as in shrimps and lobsters. Indeed, at their base the most anterior pairs of gills in a blue crab "face" forward.
Contrary to widespread popular belief, crabs can walk forward or diagonally, and some species do so quite often. But usually crabs move sideways, particularly when hurrying. The attachment of one pair of chelipeds and four additional pairs of thoracic legs within the short space available at the side of a crab favors sidewise movement over forward movement. When a crab runs sidewise, the legs on the leading side pull the body by flexing, while the legs on the trailing side push the body by extending.
One genus, the semiterrestrial ghost crab, Ocypode, can run at great speed. In tests by Dr. Dennis R. Hafeman and Dr. J. I. Hubbard, the species Ocypodc ceratoplithalrna ran at an average speed of 1.825 meters per second, or over four miles per hour, on the firm sand of a tidal beach. When on the hard deck of a ship, the crabs ran even faster, the average speed being 2.33 meters per second, or 5.2 miles per hour. These are the highest recorded speeds for any crustacean. During the tests the crabs did not use their last (fifth) pair of thoracic legs or their chelipeds, except for balancing. The second, third, and fourth pairs of thoracic legs did the moving, with the second and fourth legs on the leading side usually being extended first, to be followed by the third leg. On the trailing side, the same sequences occurred, but with a phase lag of about a third of a cycle.
Some observers have reported that when Ocypode is running, it does so with one side leading for a while. The crab stops abruptly, rotates its body, and then runs with its other side leading. The process of rotation is repeated. In this way, the flexor and extensor muscles of the legs on each side are alternately used and rested.
In the anterior portion of the cephalothorax of a crab are the mouth parts, grouped around the opening to the esophagus. These mouth parts are generally similar to those of shrimps and lobsters. The outermost pair is the third maxillipeds, used for holding food. Under and in front of these are two more pairs of maxillipeds and two pairs of maxillae, also used for holding food, and a pair of mandibles, or jaws, which push the food into the esophagus.

12.3.2. Digestive System
The foregut of crabs, like that of lobsters, has in its walls many ossicles, or small hard plates and projections, that articulate with one another in a complicated way and serve as a place of attachment for muscles that move the foregut. According to American biologists Robert Pyle and Eugene Cronin, the blue crab has in or associated with its foregut at least 50 ossicles and over 80 muscles. These effect a churning action of the foregut and a grinding of the gastric mill that break down particles of food that have been swallowed. The gastric mill of crabs resembles that of lobsters in consisting of one dorsal and two lateral teeth situated at the constriction that separates the large anterior chamber of the foregut from the smaller posterior chamber.
The midgut originates approximately where ducts from the midgut glands enter the posterior chamber. Behind this chamber, the midgut appears as a small tube, scarcely three- eighths of an inch in length in a full-grown blue crab.
The midgut glands consist of three pairs of lobes, one pair extending forward and to the sides, a second pair extending laterally toward or over the gills, and a third pair leading back toward and, in some species, into the abdomen. The midgut glands may fill much of the body cavity, although their extent at any one time depends largely upon their content of food reserves and water.
As in shrimps and lobsters, digestion of food in crabs takes place partly in the anterior chamber of the foregut, partly in the posterior chamber, and partly within tubules of the mid- gut glands. A bristly filter in the ventral wall of the posterior chamber prevents all but the most finely divided material' from passing up the ducts into tubules of the midgut glands. Since the lumen of the midgut glands is continuous with that of the midgut, these glands are diverticula of the midgut.
In the blue crab, another diverticulum arises from the mid- gut just behind the posterior chamber. A pair of tubes, known as midgut ceca, runs from the dorsolateral surface of the midgut forward and laterally, ending in coils that lie just above the first large lobe of the midgut glands. These ceca are translucent and difficult to see in dissection. Lining the lumen of the midgut ceca are cells like those lining the midgut. Both groups of cells probably function in the absorption of food.
The hindgut makes up the remainder of the digestive tract. It runs between the lobes of the mid-gut glands, under the heart, and into the abdomen, where it follows a straight course to its posterior opening, the anus. Only a slight swelling is present in the most posterior portion of the hindgut, hardly enough to-justify calling, this region a rectum. The entire hindgut is lined with chitin.
In the second or third abdominal segment, the hindgut of the blue crab gives rise to a cecum. From its origin on the left side, the cecum runs forward and over the hindgut, terminating in closely packed coils on the right side. The function of this cecum is not clear, but this organ may be involved in the regulation of salts in the hemolymph when a crab is exposed to dilute media.

12.3.3. Circulatory System
In a crab, circulation of blood takes place much as in a shrimp or lobster. Arteries carry blood dorsally from the heart forward into the head and viscera and backward into the abdomen. A sternal artery carries blood ventrally, where main branches direct it both forward and back. Further branching of main arterial vessels leads the blood into thin- walled capillaries, where exchange of gases and foodstuffs between blood and tissues can occur. The blood collects in venous sinuses, goes to the gills, and then enters the pericardial sinus surrounding the heart. Here, when the heart relaxes and its three pairs of ostia open, the blood enters the heart. In keeping with the breadth of a crab's body, the heart is broad, filling much of the pericardial sinus, or pericardium. In the small tail of a crab, the ventral abdominal artery is of relatively small size compared with the same artery in the muscular tail of a lobster.

12.3.4. Nervous System
The nervous system of all crabs, except: the most primitive, has undergone a high degree of fusion. All ventral ganglia are fused into a single thoracic ganglionic mass, which lies near the floor of the cephalothorax and through which the sternal artery descends. From the periphery of the thoracic ganglionic mass, nerves radiate out to the appendages all the way from the mandibles to the last thoracic legs. An abdominal nerve emerges posteriorly at the midline and supplies the muscles and appendages of the tail.
Connecting the thoracic ganglionic mass with the brain, or supraesophageal ganglion, are the two long, large nerves that pass on either side of the esophagus and are known as the circumesophageal connectives. Slight swellings on the connectives mark the position of the stomatogastric, or connec-tive, ganglia that supply nerves to the foregut. The tritocerebral commissure links the two connectives in crabs, as in shrimps and lobsters.
The kidneys of crabs lie on the interior ventral surface of the body, just posterior to a position between each antenna and the corresponding eyestalk on the same side. Due to their color, which is pale green, yellow, or green-brown, the kidneys are also called green glands; due to their position on the second, or antennal, segment, they are also often called antennal glands.

In structure, the antennal glands of crabs are similar to those of lobsters. There is a glandular portion, which secretes urine and regulates salts, and a large, many-lobed, thin- walled bladder, in which urine is temporarily stored. The main lobe of the bladder lies above the glandular portion of each kidney, but the remaining lobes extend out in several directions. Because of the delicacy of the lobes, it is almost impossible to see them unless they are fixed in alcohol or injected with India ink or a powdered dye, such as carmine. Urine passes from the glandular portion of each antennal gland upward into the bladder and then to the exterior via a duct. The opening of the duct, which lies at the base of the antenna, is covered by a calcified, movable cover, called an operculum.

12.3.5. Respiratory System
The gills of crabs differ from those of lobsters, where each gill consists of many filaments arranged, plume-like, around a central axis. In crabs two rows of closely set, leaf-like plates or lamella are attached to the central axis of all or, in same species such as the blue crab, all but one pair of gills. In the blue-crab one anterior pair of the gills has only one row of lamella. Gill of crabs known as phyllobranchs, after the Greek words phylion, meaning leaf, and branch, meaning gill. There are eight gills on each side of a blue crab's body.
Water enters the branchial chambers of crabs primarily through an anterior opening above the base of each claw, or cheliped, and to a much less extent through openings at the base of the other thoracic legs. In the blue crab, when the chelipeds are raised and held forward, the opening at the base of the chelipeds is very large and nearly circular. When the chelipeds are folded against the body, the opening is a wide slit, which becomes narrower when the third maxillipeds are brought close to the midline, for a flange at the base of each third maxillipeds reduces the width of the slit. Bristle-like setae arising from the basal portion of the cheliped filter some of the water entering the slit.
In crabs, the opening at the base of the last pair of thoracic legs may or may not be important in the entry of water. According to Dr. Arudpragasam and Naylor, who studied pathways of gill ventilation in several species of crabs, the more flattened and shortened the body of a crab and therefore the more it diverges from the elongated body and laterally facing gills of a lobster or shrimp, the more important in the entry of water are the anterior openings at the base of the chelipeds and the less important are posterior openings.
As in shrimps and lobsters, the current of water through the branchial chambers of crabs is maintained by the beating of the gill bailer, which lies in the channel at the anterior exhalant opening of each branchial chamber. After entering through the openings at the base of the chelipeds and, to a less extent at the base of the thoracic legs, the water passes under the gills, up between the gills, over the gills, and out through the exhalant aperture. Periodically, as a result of reversal in the action of the gill bailer, the direction of the respiratory current is reversed. This aids in cleaning the gills of debris and tends to divert water back over the gills that lie in the posterior part of the branchial chambers.

12.3.6. Reproductive System
The ovaries of a female blue crab are connected to each other just behind the foregut and extend forward and backward through the body. Thus, in blue crabs, as in American lobsters, the ovaries appear roughly H-shaped. In early stages of its development, each ovary of a blue crab is thin and white, with a short lateral arm. It still appears this way immediately after the female has shed her shell and, as a soft crab, has mated. But ovarian growth starts soon thereafter and, several months later, results in a very large ovary, which is orange because the eggs are full of orange yolk. Each ovary now may extend far out to the side of the body and into the first abdominal segment.
From the ovaries, paired oviducts run forward and downward for a short distance, then widen to form an oval-shaped structure known as a seminal receptacle.. Here are stored the sperms that the female blue crab receives from the male blue crab during mating. Each seminal receptacle slants backward and downward and then narrows into a short tubular vagina, which runs ventrally to an opening on the sixth thoracic segment. Although the oviducts and the dorsal portion of the seminal receptacles are soft and unlined, the ventral portion of the seminal receptacles and the vagina are hard, being lined with chitin. At the time of ecdysis, this lining is shed, along with other portions of the exoskeleton.

In an immature blue crab the seminal receptacles are small and white. Yet, in a mature crab immediately after copulation, the seminal receptacles are enormously distended, at times equal in size to the heart; and they are pink in color, due to the presence of a gelatinous "sperm plug" that keeps the sperms secured within the receptacles. Later, after the sperm plug has been absorbed, the receptacles are again white.
At the time of ovulation, when ripe eggs are released from the ovaries and move down the oviducts, sperms fertilize the eggs either within the oviducts or within the seminal receptacles. When fertilized eggs emerge from the vagina, they be come attached to the pleopods of the female and remain there until ready to hatch into the first larval stage. Yet many sperms remain within the seminal receptacles and many eggs within the ovaries, so usually a second "laying" occurs, after which the ovaries appear collapsed and grey or brown in color as they begin to degenerate. Yet even now, enough sperms remain within the seminal receptacles to fertilize several more batches of eggs, were the eggs able to ripen.
In an immature blue crab the seminal receptacles are small and white. Yet, in a mature crab immediately after copulation, the seminal receptacles are enormously distended, at times equal in size to the heart; and they are pink in color, due to the presence of a gelatinous "sperm plug" that keeps the sperms secured within the receptacles. Later, after the sperm plug has been absorbed, the receptacles are again white.
At the time of ovulation, when ripe eggs are released from the ovaries and move down the oviducts, sperms fertilize the eggs either within the oviducts or within the seminal receptacles. When fertilized eggs emerge from the vagina, they be come attached to the pleopods of the female and remain there until ready to hatch into the first larval stage. Yet many sperms remain within the seminal receptacles and many eggs within the ovaries, so usually a second "laying" occurs, after which the ovaries appear collapsed and grey or brown in color as they begin to degenerate. Yet even now, enough sperms remain within the seminal receptacles to fertilize several more batches of eggs, were the eggs able to ripen.

In the male blue crab the testes consist of a pair of slender, convoluted, opaquely white arms lying on the dorsal surface of the midgut glands. Medially, the terminal portion of each arm passes around the posterior end of the foregut and joins with the other arm to form a short cross-bar. Just anterior to the cross-bar a tiny tube, the vas efferens, connects each arm of the testes with a much-coiled vas deferens. The vas efferens is difficult to find since it is concealed within the testis and the coils of the vas deferens.
The vas deferens consists of several portions. The first known as the anterior vas deferens, is white and tightly coiled and lies close to the middorsal line between the foregut and the heart. Here the sperms are gathered in egg-shaped bundles, called spermatophores, and stored. In the second .portion, the median vas deferens, the coils form a large mass and appear pebbled pink, due to their content of material that subsequently is deposited in the seminal receptacles of the female during copulation and forms a sperm plug.

The third portion, the posterior vas deferens, is long, convoluted, greenishly translucent, and almost empty except during passage of the spermatophores. The final portion, the penis, is a short, translucent tube at the base of the last pair of thoracic legs. The penis lies permanently within a groove in the first pair of abdominal appendages, the copulatory pleopods. These, in turn, are inserted into the seminal receptacles of the female during copulation. Also fitted into the groove on the copulatory appendages of the male is his second pair of pleopods, which during copulation act as pistons to push the spermatophres along the groove, where they break up and release the sperms.

The T-shaped abdomen and elongated, grooved copulatory pleopods of the male blue crab are his most distinguishing external sexual characteristics. In contrast, the abdomen Of the mature female blue crab is broad and rounded, and her pleopods are relatively short and fringed with hair-like setae, to which eggs are attached during development. Coloration also serves to separate the sexes. Normal coloration of both male and female blue crabs consists of a dark blue-green or gray-green carapace and bright blue and blue-green legs, with scarlet markings. Except for the appendages and the female abdomen, the underparts are white. In the male, the greater portion of the chelipeds, or claws, is blue-or gray-green, with dull purple "fingers," whereas in the female there is more blue on the chelipeds and the "fingers" are bright red.
It may interest the reader to learn that just as there are blue American lobsters, so also there are blue blue crabs. Some years ago a specimen of blue dab' was described as having a carapace of robin's egg blue and appendages of pale blue with traces of pale red. The under surface of the body was white. Also, just as parti-colored American lobsters exist, so do parti-colored blue crabs. One such specimen was described as being gray on the left side and brownish on the right. A tendency toward albinism occurs in blue crabs, as it does in American lobsters.

MOLLUSCA

13.1. General characters

·         Molluscs are essentially aquatic, mostly marine, few freshwater and some terrestrial forms.

·         The body is soft, unsegmented, bilaterally symmetrical and consists of head, foot, mantle and visceral mass.

·         The body is clothed with a one layered often ciliated epidermis.

·         Body is commonly protected by an exoskeleton calcareous shell of one or more pieces, secreted by the mantle.

·         Head is distinct, bearing the mouth and provided with eyes, tentacles and other sense-organs excel: the Pelecypoda and Scaphopoda.

·         Ventral body wall is modified into a muscular flat or plough-like surface, the foot which is variously modified for creeping, burrowing and swimming.

·         Mantle or pallium is a fold of body wall that leaves between itself and the main body mass, the mantle cavity.

·         Visceral mass contains the vital organs of the body in a compact form taking the form of a dorsal hump or dome.

·         Body cavity is haemocoel. The true coelom is generally limited to the pericardial cavity and the lumen of the gonads and nephritic.

·         Digestive tract is simple with an anterior mouth and posterior anus but in gastropods, scaphopods , and cephalopods the intestine becomes U-shaped brin

·         ging the anus to an anterior position.

·         Pharynx contains a rasping organ the radula except in Pelecypoda.

·         Circulatory system is open except in cephalopods which show some tendency towards a closed system.

·         Respiratory system consists of numerous gills or ctenidia usually provided with osphradium at the base. Lung is developed in terrestrial forms.

·         Excretory system consists of a pair of metanephridia which are true coelomoducts and communicate from pericardial cavity to the exterior by nephridiopore.

·         Nervous system consists of paired cerebral, pleural, pedal and visceral ganglia joined by longitudinal and transverse connectives and nerves.

·         Sexes usually separate (dioecious) but some are hermaphroditic. Fertilisation is external or internal.

·         Development is either direct or with metamorphosis through the trochophore stage called veliger larva.

Unio or Lamellidens or Anodonta

13.2. Unio or Lamellidens or Anodonta

Class - Pelecypoda
Distribution
The Freshwater Mussel is a familiar representative of the phylum Mollusca. The family Unionidae is widely distributed all over the world and includes nearly all the large freshwater mussels or clams. The family consists of several genera and nearly 1,000 species of which a good number are represented in India. The commonest species in England is the Swan Mussel (Anodonta cygnea). The types commonly dissected in India are Unio and Lamellidens marginalis. The description that follows will apply in general to almost any freshwater mussel.

13.2.1. Habits and Habitats
The freshwater mussels are found in ponds, lakes, rivers and streams, some in quite and others in flowing waters. They occur more abundantly in waters containing lime as this material is necessary for the production of their shell. They live nearly buried in the mud or sand at the bottom, with only the posterior tips of their shell valves exposed. They may also occur wedged in between the rocks and stones, with the valves slightly spread, and the two siphons exposed. They burrow and crawl slowly by extending their large muscular and ploughshare like foot between the two valves, or through the gape of the shell, and larvae a furrow to mark the path they have followed. They may migrate to shallow places by night and retire to deeper places by day, and may change the habitats with the seasons.

13.2.2. External Features
Shape and size
The size varies from 5 to 10 cm. in length. The body is soft, unsegmented, bilaterally symmetrical and battened from side to side. It is sandwiched between the two valves of tie shell. When viewed from the side to: likely looks oval, with a blunt anterior end, the slackest part being in the middle of the body, near the dorsal side becoming gradually thinner toward the ventral edge.

Shell
The animal IS completely surrounded by a hard calcareous shut. It is brownish in colour.
Valves: The shell consists of two separate, equal and lateral pieces called values, covering the right and left sides of the body, respectively. The shill of a mussels made of two vales, is called bivalve as distinguished from the univalve shell of a snail, made of a single pied. In a smile flew, the shell is opal, the anterior and being rounded and the posterior mere pointed.
Hinge-ligament: Write two valves of tie shill are united tomato along the aerial id: in a straight bilge line by an external, browns tough, illicit - and note' calcareous or horny hinge-ligament made of conohlolin. The gape of the shell is ventral. The Elastic ligament draws the valve: together dorsally and causes them to gape ventrally.
Umbo: Dorsally, in front hinge ligament of the dingo and nearer the anterior dorsal pillar end, there is a whitish knob-like pert re swelling in each valve, called the umbo, which are the thickest and the oldest portion of the shell. It is the first part or the shell the develop in the late veliger larval stage. It is usually corroded by the action of carbonic and hurdle acids in water. Since entitle edge tie umbo is directs d anteriorly, it is possible to determine the right and left posterior
Lines of Growth: The outer surface of each shell valve presents a number of concentric lines around the umbo as centre and running parallel to the free large of the shell.
These are the lanes or benign of growth, representing intervals between successive growth stag's.

The shell increase in size by the deposition of new rings around the outer rim of the shell each year. Rings that are far apart indicate periods of rapid growth, owing to a plentiful food supply; periods of restricted growth are represented by close-fitting rings. The rings representing the dormant periods of the winter months are more conspicuous so that it is possible to estimate the age of a clam by these rings.
Hinge teeth
In Unio and Lamellidens, the inner surface of each valve possesses dorsally along the hinge line, small sharp ridges and teeth-like projections separated by grooves or sockets. These are known as hinge teeth. The teeth of one valve fit into the corresponding sockets of the other, thus holding the valves from slipping out of the position. They differ in number, shape and degree of development in different types of mussels and are entirely absent in Anodonta. The hinge-teeth close the shell with amazing precision and the margins of the shell articulate perfectly like the edges of a well-made locket. This precision is necessary because, if the shell valves did not join tightly, the clam would be much more vulnerable to its many enemies.
Muscle scars
The inner surface of each valve also bears characteristic markings or impressions, indicating the former attachment of muscles. Near each end, anteriorly and posteriorly, is a large and oval scar of the adductor muscle, that of the posterior muscle being larger than that of the anterior. Near the impression of the anterior adductor muscle are two small impressions, the dorsal and posterior one left by anterior retractor muscle, and the ventral and posterior by protractor muscle. A small impression of a posterior retractor muscle also lies dorsal to the impression of the posterior adductor muscle. Running inside and parallel to the free ventral margin of the valve, from one adductor impression to the other, is a fine groove or line, the pallial line, which marks the attachment of the muscle fibres from the edge of the mantle (retractor pallial muscles).
If a clam is molested, the foot at once withdraws in- side the shell, the two valves of which are slowly and lightly shut by the powerful anterior and posterior adductors, thus barring the entry of in-truders. The attempt to pull apart the two valves of the shell may not succeed unless a thin-bladed knife is first inserted through the gape of the shell to severe the large adductor muscles. The starfish, however, has a novel way of opening the shell valves. It circumvents the clam, attaches its tube feet to the shell valves and exerts a steady pull. Sooner or later, the adductor muscles of the clam become exhausted and relaxed so that the shell opens.

13.2.3. Microscopic structure of shell
When viewed in a cross section, the shell presents three distinct layers: periostracum, prismatic layer and nacreous or pearly layer.
Periostracun It is the outermost, greenish-brown, thin, translucent, horny layer, formed by a chitin-like horny organic substance, the conchiolin. It is secreted first by the edge of the mantle. It is rough and serves to protect the underlying calcareous layer, from being dissolved by the corroding action of weak carbonic acid in water. It gives the exterior of the shell most of its colour. It is often eroded or worn away from older parts of the shell, like umbo, where the median prismatic layer becomes exposed.
Prismatic layer. The middle or prismatic layer is also secreted by the mantle-edge. It consists of minute prisms or crystals of calcium carbonate, prismatic separated by thin layers of chiolin, arranged perpendicularly nacre to the surface of the shell It gives strength to the shell.
Nacreous or pearly layer The innermost layer called nacre or "mother-of-pearl', is secreted by the whole outer surface of the mantle and present a smooth, iridescent or lustrous surface.

It consists of alternate of the shell and the mantle, layers of calcium carbonate and conchiolin, laid down parallel to the surface of the shell. The mantle deposits nacreous layers over any irregularities that occur either in the shell or over loose particles that may lodge in the mantle itself. The result is the formation of a pearl.
The proportion of the CaCO3 in the entire shell varies from 89-99, whereas that of phosphate of lime from 1-2%. The chitin like-horny organic base, called conchiolin provides a sort of membranous framework.

13.2.4. Pearl formation
A pearl is the result of any injury to the mollusks. It is secreted by the mantle as a means of protection against some foreign body. Whenever a foreign body, such as a grain of sand or a small parasite, such as a larval stage of a fluke, gets between the mantle and the shell it becomes enclosed in a sac of mantle epithelium which is thus irritated. The irritation stimulates the mantle epithelium to secrete thin concentric layers of mother of pearl around the foreign body. The amount of deposition is in direct proportion to the degree of irritation, At the end of several years, a pearl will be formed. Pearls are often found in clams and edible oysters but these are not nacreous and therefore or little value. The most precious pearls are found in the pearl oyster (Pinctacla vulgaris), which is closely allied to the freshwater mussel. The Japanese have developed a technique of producing pearls artificially by inserting foreign bodies such as glass beads, into the mantle of oysters which are retained in wire cages or crates until pearls are produced which can be later removed and sold in the market. It requires 3 to 4 years to produce a pearl of considerable size but a large one requires 7 years. Cultured pearls are genuine pearls but less valuable than uncultured pearls which can be identified by experts. Imitation pearls are beads coated with an iridescent substance called pearl essence that is obtained from the scales of fish.

13.2.5. Mantle or Palliun
Beneath the shell, the soft body of the mussel is enveloped in a thin, semi-transparent and soft covering of skin, called the mantle or pallium, which secretes the shell. It also consists to lateral halves, the mantle lobes or folds, which are continuous dorsally. Each mantle lobe is a thin sheet of tissue, closely applied to the inner surface of the valve. The ventral free border of each mantle lobe is thickened and contains muscles which insert upon the pallial line.

At the posterior end, the mantle lobes are thickened, muscular and form two short tubes or siphons. The current of water enters through the ventral incurrent or inhalent siphon and leaves through the dorsal excurrent or exhalent siphon. The inhalent siphon is wider, with a fimbriated or papillated margin and formed simply by coming together of the two mantle lobes. The exhalent siphon is narrower, with smooth margin, and formed by the fusion of the two mantle lobes. On the postero-dorsal side, the two mantle lobes also form a dorsal mantle pore.
Histologically, the mantle consists of—

·         an outer columnar epithelium beset with numerous unicellular glands secreting nacre,

·         a middle fibrous connective tissue, and

·         an inner ciliated epithelium containing mucus-secreting cells.

13.2.6.Mantle cavity
The space enclosed between the two lobes of the mantle is the mantle or pallial cavity. On removing one mantle lobe, the mantle cavity and its organs (visceral mass, gills, foot, etc) are exposed.
Visceral mass
The soft body or visceral mass occupies the dorsal parts of the mantle cavity. It is dark in colour and contains various organs including the digestive, circulatory, excretory, and reproductive systems. In a freshly-killed animal, a greenish-brown digestive gland is visible in the antero-dorsal region, a pericardial cavity containing the heart in the mid-dorsal region and dark-coloured paired kidneys below the pericardium.
Head
The freshwater mussel lacks a distinct head as it would not be of much use to an animal that lives with its anterior end buried in mud. The eyes and tentacles are absent. The large mouth opens beneath the anterior adductor muscle, bordered by a pair of broad, lamellar labial palps on each side, while the anus' lies above the posterior adductor muscle.
Gills
A pair of long, double plate-like gills hangs freely in the mantle cavity, one on each side from the visceral mass. The gills have a sieve-like structure, perforated by minute pores and covered by cilia. Their line of attachment to the visceral mass forms a continuous horizontal partition, dividing the mantle cavity into a large ventral infra-branchial chamber, and a small dorsal supra-branchial chamber.
Foot or podium
The antero-ventral hatchet-like prolongation of the visceral mass, hanging down in the mantle cavity forms a large, muscular, extensile foot or podium which is adapted for burrowing. It is laterally compressed and terminates below into an elongated keel. The thick basal part of the foot contains a portion of alimenty canal, the digestive gland and the gonad. The foot can be extended by blood pressure and by the muscular action of a pair of pedal protractor muscles. It can be withdrawn into the shell by the action of anterior and posterior retractor muscles.

13.2.7. Locomotion
The foot is the chief locomotory organ and its size and shape are always changing in the living animal. The wedge-shape foot is adapted for progression in the mud or sand at the bottom of the river of lake where it lives. In a buried clam, the shell valves slighty agape ventrally and through this opening the fleshy foot protrudes and burrows through the mud like ploughshare. As the mussel wants to move, the pointed foot is extended forward, as far as possible into the mud by the contraction of the protractor muscles. An influx of blood now takes place into the cavity of the foot so that its tip swells up, becomes td and acts as anchor. Sphincter muscles round veins prevent the return of blood from foot. Next, contraction of the retractor muscles pulls the body of the mussel forward through or deeper into mud. The blood is forced out of the foot which 1'ns down again and can be extended forward in the mud. The repetition of these movements of the foot results in a slow progression of the animal, while a narrow wedge-shaped path is left behind.

13.2.8. Body Wall
The whole external surface of the body is covered by a single layer of epithelial cells, which is mostly ciliated, especially on the gills, the labial palps and the internal surface of the mantle. The skin of the foot contains glandular cells, while glandular and sensitive cells are abundant on the mantle edge. Beneath the epidermis, internal spaces of the visceral mass are occupied by the connective and muscular tissues.

13.2.9. Body Cavity
The general body cavity is a haemocoel filled with blood. The true coelom is schizocoelic and greatly obliterated by the connective tissue, unstriped muscle fibres and blood sinuses. It is represented only by three small cavities—(l) a single ovoidal chamber, the verlcardium, which lies dorsally, containing the heart and a part of the intestine, and lined by the coetomic epithelium; (2) the gonocoels or the cavities of the gonads; and (3) the urocoels or the cavities of the excretory organs.
Musculature
The muscles are mainly of the slow contracting, unstriped type and arranged in distinct bands or sheets. The two shell valves are closed by the contraction of two large, strong, cylindrical transverse muscles, situated one close to either end dorsally and passing across the body from one valve to another. They are called anterior and posterior adductor muscles. When these muscles relax, the elastics hinge ligament opens the valves. Near these muscles, are two smaller muscles, the anterior and posterior retractor muscles, which run from the foot to the shell and serve to withdraw the foot, during locomotion. A small protractor muscle, close behind the anterior adductor, serves to compress the visceral mass, thus causing the protrusion of the foot. The complex intrinsic muscle of the foot also serves as a protractor of that organ. The delicate palliàl muscles, inserted upon the shell all along the pallial line, serve to retract the edge of the mantle.

13.2.10. Respiratory System
Respiration is aquatic and carried on simultaneously w the feeding process. The respiratory organs are the gills the mantle.
Gills or ctenidia
The freshwater muscle respires the oxygen dissolved in water by a pair of gills or cienidia or branchiae. On each side of the foot is a single gill, hanging the mantle cavity between the mantle and the visceral ma like a flattened, plate-like structure. The great length of i gills has become possible due to the large siz of the man cavity into which they extend antero-posteriorly.
Structure of a ctenidium
Each ctenidium is compos of two more or less rectangular plates or laminae, one innerand other outer. Each gill lamina is a hollow double-fold, formed of two thin parallel plates or lamellae, an inner and an outer one, united together at their anterior, ventral and posterior edges, but free dorsally. Each gill lamina thus forms an elongated narrow bag, opening dorsally into a supre branchial chamber. The lamellae are joined together by vertical cross partitions or inter-lamellar junctions, so that the thin space between the two lamellae is divided, at regular intervals, into a series of vertical narrow compartments or water tubes. The water tubes or each gill lamina are closed ventrally, but join a common supra-branchial chamber, dorsally. Each gill lamella consists of a large number of close-set, thin, vertical gill bars or gill filaments, which impart vertical striations to the outer surface of the lamella. The adjacent gill filaments are connected by small bridge-like, horizontal bars, the inter- filamentar functions, which impart horizontal striations to the laminae. The filaments of both the lamellae are continuous at the free ventral edge so that each lamina appears V-shaped in a transverse section, and the ctenidiurn of each side resembles a W. The gill, lamellae have a porous or sieve-like structure, being perforated by minute but frequent openings, the inhalent ostia, bounded by filaments and their junctions and leading into the water tubes. Thus, the structure, of each gill-plate is very complex like a piece of basket-work.

The ctenidia of the mussel are of the eulamellibranch type. Each ctenidium is bipectinate, with a central ctenidial axis, from which filaments arise in two rows, one on either side. The filaments of each row are folded in the middle to appear V-shaped in section. Of the two arms of V, one is descending, the other ascending. At the angle of the fold each filament is notched. The notches of all filaments form a continuous food groove, that extends the whole length of the underside of each lamina of the ctenidium.
The gill filaments are composed of connective tissue. They are strengthend by chitinous rods and covered by a ciliated epithelium. The cilia are of three types, those present on the outer ridge-like faces of the filaments are called frontal cilia, those on lateral parts are lateral cilia while those lying in between are the latero-frontal cilia.
Attachment of ctenidia
The mode of attachment of Sills to the body determines the course of water current in the body. The gill axis or ctenidial axis remains fused to the dorsal wall of the mantle cavity throughout, but becomes free near the posterior end of the body. The outer lamella of the outer lamina is attached to the mantle throughout. The inner lamella of the inner lamina is attached to the viscero-pedal mass anteriorly, becomes free in the middle region, and fuses with that of the other gill posteriorly. In this way, a suprabranchial chamber is formed above each lamina in the anteriol and middle regions, while in the posteriar region, the dorsal edges of all the four laminae form a sort of continuous hon. zontal partition separating the infra-branchial chamber of thc mantle cavity below from a supra-branchial or cloacal cavity above. The inhalent siphon leads into the infra-branchia chamber of the mantle cavity, while the cloacal cavity leads t the outside through the exahient siphon.

Blood supply of gills
The gills receive venous blood in the kidneys, through the afferent branchial vessels. From s the oxygenated blood is returned to the auricles through efferent banchial vessels.
Corse of water current
The constant bearing of lateral a covering the gill filaments draws a continuous current of water through the inhalent siphon into the mantle cavity. The present on the inner mantle surface and labial palps also p in production of the water current. The current can easily be demonstrated by placing a few grains of powdered mine in the neighbourhood of the siphons. From the infrauchial chamber, the water enters the gills through the ostia. Within the gills, it flows up the water tubes to enter the supra-nchial chambers, which run one above each gill lamina and and posteriorly into the cloacal chamber. From here, the rent passes, through the exhalent siphon. It has been mated that water passes, through the gills of a mussel of rage size, at an average rate of 2 litres an hour.
Physiology of respiration
Respiration takes place through the walls of the gills. The blood, circulating in the foot water current outer gill-lamina mantle cavity branchial vessels in the interlamellar junctions of the gills, transfers CO₂ to the surrounding water and receives 0₂ from it. The water entering the mantle cavity carries with it not only oxygen but also the minute food particles, which are moved on by cilia towards the mouth. Similarly, the outgoing water carries with it not only the faecal matter but also the excretory and reproductive products. Probably the main function of the gills is to produce the food current rather than respiration which is shared more by the mantle.

Mantle. The surface of mantle is always bathed in water, and it is devoid of a cuticular covering which might hinder diffusion of gases. The mantle is also richly supplied with blood which is sent directly to the heart. It is, therefore, probable that it serves as an important supplementary respiratory organ.

13.2.11. Digestive System
The digestive system consists of an alimentary canal and a paired digestive gland.
Alimentary canal
It is a long and coiled tube of varying diameter, consisting of the following parts.
Month and labial palps
The mouth is a transverse slit, lying in the middle line between the anterior adductor muscle and the foot. It is bounded on each side by two, somewhat oval, fleshy flaps, the inner and outer labial palps. The cilia on the surface of these palps drive the food particles into the mouth. The two labial palps on each side enclose a ciliated oral groove leading into mouth. The mouth lies between two ridges or lips, the anterior lip connecting the two outer palps, and the posterior lip connecting the two inner palps of the opposite sides. The mouth has no teeth.

Oesophagus
The mouth leads directly into a short tube, the oesophagus. A radula is absent, being useless to the animal that feeds only on minute organisms. The inner wall of oesophagus is ciliated.
Stomach and style sac
The oesophagus opens dorsally into a wide, thick-wailed oval sac, the stomach, surrounded by a large paired digestive gland or liver and connected with it by several ducts. The posterior end of stomach gives off ventrally a tubular diverticulum, the pyloric caecurn or the style sac. It contains a transparent, solid, flexible and gelatinous rod, the crystalline style. It is secreted in concentric layers by the ciliated epithelium lining the style sac and contains a mucoprotein and a cellulose and starch digesting enzyme. The cilia if the style-pouch rotate and move the style forward, so that its free end, projecting into the stomach, is constantly rubbed against a special portion of stomach wall, called the gastric shield. Thus, the head of the style is constantly worn away and its substance is mixed with the contents of the stomach.
Intestine
The intestine arises from the stomach floor in front of the style sac. It runs ventrally into the foot, where it is coiled upon itself through the visceral mass, much of which is the yellow-coloured branched gonad. It then runs dorsally again and is continued into the rectum.
Rectum
The rectum, or terminal part of the intestine, runs posteriorly through the pericardium, traversing the ventricle which is actually wrapped around it. Finally, it ends in an anal papilla which opens in the cloacal chamber above the posterior adductor muscle by the anus. Internally, the rectum has afolded mid-ventral ridge, like the typhiosole of the earthworm.
Digestive gland
The paired digestive gland or liver of the freshwater mussel is a large, dark brown or green gland of an irregular shape, surrounding the stomach. It is made of highly branched tubules, lined by columnar epithelial cells of unequal height and containing basal nuclei. It opens into anterior end of the stomach by many ducts, which are lined with ciliated epithelium. The gland not only produces digestive ferments, but its cells readily ingest and break down solid foot particles, digest proteins and fats intraceliularly, and absorb carbohydrates. The gland is also regarded to be excretory by some people.
Food and feeding mechanism
The food consists of diatoms Protozoa other planktonic micro-organisms and organic detritus brought in by the respiratory water currents. The mussel is typical filter feeder and the ctenidia also serve the function to obtain food. It is a sluggish creature, incapable of actively seizing its prey. Therefore, it obtains its food through current set in water by ciliary movements, in the manner of sedentary organisms. Much like the sessile sponge, mussel must pump large quantities of water through its body, in order to extract food and oxygen and to get rid of the wastes.

The beating of lateral cilia borne on the outer surface of gill fluent and coca precut on tike inner surface of tile mantle draws a constant water current, through the indolent siphon into the mantle cavity with microscopic plants lad animals suspended in it.

The particles are strained out as the water passes through the gills. The heavier sand particles are simply dropped from the surface of the galls to the edge of the mantle, driven backwards by the cilia on tile mantle, and expelled out posteriorly. The lighter food particles are thrown by the latero-frontal citron to the outer lamellar surfaces, where they become entangled in mucus secreted by the galls. The food-laden mucosa mass from both sides of each gill Lima are moved ventrally by beating of the frontal calla into the food groove at the lower edge of the gill lamina. Cilia of the food groove drive tie food I parolees forward towards tie mouth When the labial palps are reached further sorting takes place, m an unknown way, according to the nature of the particles. The large and indigestible particles, under the influence of the cilia working along rejection paths, drop down and get removed from the mantle cavity with the outgoing circulation on the mantle at the posterior end throngs the incurring siphon. The call and digestible particles are (tarried straight into the mouth, along the deep collate groove between time two labial palps, leading to a corner of the mouth.
Digestions
Tie physiology of digestion in freshwater mussels is not known. Its knowledge is mainly basal on be smiles of some marine clams. The food is mostly digested and parry absorbed in the stomach. As the feed is microscopic, tie digestion is mostly intercellular by phagocytes lining tie alimentary canal and by wandering leukocytes which play a truth part in the teleport of food all over the body.
Food from stomach esters the digestive gland, the cells of which have a surprising power of ingests solid particles, lid absorbed digesting protists and fats intracellularly, carbohydrates. Folds of the stomach wall around the apertures of the ducts of digestive gland permit only tie finest food particles to enter the gland. According to Owen (1955) the duds of the digestive gland show two kinds of track, non- ciliated for receiving tie food particles and ciliated for sending time undigested wastes back into the stomach. Extra-cellular digestion also occurs in tile stomach. A digestive fluid is secreted by to digestive gland and liberated m the stomach by ducts. It is said to contain the enzyme amylase. The crystalline cone also helps in extra-cellular digestion in two ways.

13.2.12.Circulatory system
The circulatory system is well developed and of the open types. It consists of (1) the heart and pericardium (2) arteries (3) sinuses, (4) veins and (5) blood. Capillaries are absent.

Heart and pericardium

Near the mid-dorsal line ant just in front of the posterior adductor muscle, there is a thin walled triangular chamber, the pericardium, which encloses the heart. The pericardium, lined by an epithelium and filled with fluid, represents a portion of coelom and communicates with the supra-branchial chamber through the kidneys.
The heart is highly contractile and 3-chambered. It consists of a single ventricle and two auricles, lying one on either side of the ventricle. The auricles are transparent, thin walled roughly triangular and distensible to a great extent. The receive blood returning from the gills and mantle. Each auricle is attached to the pericardial wall by a broad base and opens into the ventricle by an auriculo-ventricular aperture, having valves opening towards the ventricle. The single median ventricle is a large, thick-walled, and muscular and tubular chamber, wrapped around the rectum, and pumping blood to the body. The muscular heart beats from 20 to 100 times per minute, so that movement of the blood is made possible.
The circulation is rather slow, but it seems to be adequate for such a slow-moving, sedentary animal.
Arteries
The ventricle pumps blood, both forward and backward, through two main arteries, known as the anterior aorta and posterior aorta, respectively. The anterior aorta runs anteriorly, dorsal to the intestine, and supplies an anterior pallial artery to the mantle, pedal artery to the foot and visceral artery to the visceral mass. The visceral artery supplies the stomach, digestive gland, intestine and gonad through the gastric, hepatic, Intestinal and genital branches, respectively. The posterior aorta runs posteriorly, ventral to the intestine and supplies the rectum, mantle, pericardium, nephridia, etc.
Sinuses
The arteries break up into a network of smaller branches in all the tissues of the body emptying into irregular cavities, the blood sinuses or lacunae, which lack the epithelial lining of true blood vessels and connect directly with veins. The freshwater mussel, therefore, has an open circulatory system.
Veins
The venous blood from various parts of the body is collected by several sinuses and smaller veins, whence it is finally collected by a large longitudinal vein, the vena cava which lies beneath the pericardium in between the kidneys. The vena cava supplies all its blood, through afferent renal veins. to the kidneys, where the nitrogenous waste is eliminated from it. From kidneys, the blood is collected by efferent ;! veins, and passes to the gill of its side for oxygenation throu longitudinal afferent branchial vein, which gives off a branb to each gill filament. From gills the oxygenated blood is returned to the auricles by efferent branchial veins. The vena cava also sends some venous blood directly to the auricles.
The mantle also serves as an important respiratory organ and it sends aerated blood directly to the auricles, through pallial veins, without having passed through the gills or kidneys.
Blood
The blood consists of colourless plasma without haemoglobin but with haemocyanin and with numerous white amotheid cells, or leucocytes, floating in it. There are no red cells as found in the blood of man and other vertebrate animals. The blood distributes oxygen and nutriment to all parts of the body and transfers CO₂ and other waste products of metabolism to the gills, mantle and kidneys.

13.2.13.Excretory System
Excretion is taken care of by—(l) a pair of kidneys or organ of Bojanus, and (2) a pericardial gland or Keber's organ.
Kidneys or organs of Bojanus
The two kidneys o nephridia are often termed as the organs of Bojanus, after the name of their discoverer. They are situated beneath the floor of the paricardial cavity, one on each side of the vena cava. They are derived from the true coelom (urocoels).
Each kidney is a long, dark, glandular tube open at both ends. It is bent upon itself like a broad U-shaped tube, with the loop posterior, the two ends anterior and the two I lying parallel and one above the other. The lower arm is bi spongy, glandular and thick-walled, forming the kidney proper, which opens anteriorly into the fluid filled pericardial c by a small ciliated reno-pericardial aperture. The dorsal a small, non-gandutar, lined by ciliated epithelium and walled, known as the ureter or urinary bladder, which anteriorly into the supra-branchial chamber by a small aperture, between the inner gill lamina and the visceral The bladders of both the kidneys intercommunicate by an aperture.

Physiology of excretion

The ventral glandular portion the kidney extracts guanin and other nitrogenous waste products of metabolism from the coelomic pericardial fluid as v the blood supplied to the kidneys. The walls of the pericardial sinus are also glandular, and supposed to secrete waste mal from the blood into the coelomic cavity.
The ciliated epithelial lining of the bladder produces outgoing current, thus carrying excretory fluid iron glandular part to the supra-branchial chamber, which ie the excurrent siphon. There is reabsorption of salts in. th kidneys, which also serve in maintaining the blood concent by removing excessive water from it.
Keber's organ
The Keber's organ or the pericardial gland is a large reddish-brown glandular mass situated in front of the pericardium. It probably helps in excretion, discharging into the pericardium.

13.2.14. Nervous System
Freshwater mussel has a muscular body, performing co-ordinated movements; but the nervous system is greatly reduced due to sluggish and sedentary mode of life and there is little evidence of a brain. It consists of—(l) a central nervous system, including three pairs of ganglia for three main regions of body (head, foot and viscera) respectively, (2) their connectives, and (3) small nerves.
Cerebro-pleural ganglia
At the base of the labial palps, just outside the corner of the mouth on either side, is placed a small yellowish, somewhat triangular cerebro-pleural ganglion, about the size of the head of the pin. The ganglia of both the sides are equivalent of the brain. They are connected with each other by a transverse cerebral commissure which passes over the cesoohagus. The cerebro-pleural ganglia supply nerves to the anterior adductor muscle, the labial palps and the anterior region of the mantle lobes. Besides, each ganglion gives off two conspicuous connectives, a cerebro-visceral connective which runs posteriorly to unite with the visceral ganglion of that side, and a cerebro-pedal connective which passes ventrally into the foot to unite with the pedal ganglion of the same side.
Pedal ganglia
The pedal ganglia lie in the foot at the one-third distance from its anterior end, dorsal to its muscular portion and just below the visceral mass. The two pedal ganglia are closely united to form a bibbed mass, which gives off nerves to the foot, its muscles and the statocysts. Each pedal ganglion is connected to the cerebro-pleural ganglion of its side by the cerebro-pedal connective.
Visceral ganglia
The two visceral ganglia situated mid-ventrally upon the posterior adductor muscles, are fused together into a flattened somewhat rectangular mass. On each side, the visceral mass gives off—(l) a dorsal pall/al nerve, a posterior pall ial nerve to the posterior part of the mantle, a posterior renal nerve to the kidney, (4) a branchial nerve to the gill, and (5) posterior adductor nerve to the corresponding muscle. Besides, it is connected with the cerebro-pleural ganglion of each side by a long, thin cerebra-visceral connective, which runs forward through the substance of the kidney just below the place of attachment of the gill lamina, giving off several small nerves to the visceral mass on its way.

13.2.15. Sense Organs
The sense organs are poorly developed due to slow, sluggish and sedentary habits. The eyes and tentacles are altogether absent.
The main sensory parts are— (1) statocysts, (2) osphradium and (3) scattered epithelial nerve sensory cells.
Statocysts
The foot contains a pair of minute hollow vesicies, the statocysts, one close to each pedal ganglion. They are innervated by the cerebro-pedal connectives. Statocyst is lined by sensory cells and contains a mass of lime, called a statolith, the movement of which stimulates the sensory cells. The statocysts are thought to be organs of equilibrium.

Osphradium
At the base of gills and on the surface over the visceral ganglia, is a pair of dark yellow patches of sensory epithelial cells forming the osphradium, which is probably used for testing the chemical nature of the water entering the mantle cavity through the inhalent siphon.
Sensory cells
The edges of the mantle-lobes are provided with scattered sensory cells, especially abundant on the inhalent siphon. They probably respond to touch (tactile) and also seera to be sensitive to light (photoreceptors).

13.2.16. Reproductive system
The freshwater clams are dioecious, i.e., the sexes are separate, but there is no sexual dimorphism. Reproduction involves a parasitic larval stage, called the glochidium in freshwater clams and the veliger in marine forms.
Gonads
The gonads, either testes or ovaries, are a pair of large, simple, racemously branched structures, lying among the intestinal coils in the visceral mass just above the foot. During breeding season, the gonads become greatly enlarged and conspicuous, when it is difficult to distinguish their paired nature. When mature, the testes are whitish and the ovaries reddish, and the two sexes can be distinguished though not very distinctly. The coelornic epithelium, lining the tubules of the gonads, gives rise to the spermatozoa in male and ova in female. The mature ovum is large, rounded, filled with a finely granular cytoplasm rich in yolk, and containing a vesicular nucleus with nucleolus.
The gonad of each side has a short duct, the vas deferens in male or the oviduct in female. It leaves the gonad from its upper side and opens into the supra-branchial chamber of the inner gill lamina by a genital aperture, just in front of the renal aperture of the ureter. There are no accessory reproductive organs.

Loligo (The Squid)

13.3. Loligo (The Squid)
A very common cephalopod and one that has become an important animal in nerve physiological research is the squid, particularly belonging to the genus Loligo.

13.3.1. Distribution
The genus Loligo has a world-wide distribution in the warmer seas. In America, Loligo pealli is one of the most common squids, found along the Atlantic coast, from Nova Scotia to Florida, while Loligo apalesens is a common sight along the entire Pacific coast of the United States. Squids are quite aboundant in the Indian coastal waters, but they hunt in great shoals in the inland waters such as the Gulf of Kutch, the Gulf of Mannar and the Palk Bay and Strait. etc. Fossil remains indicate that squids were once very abundant during prehistoric times.
Habits and Habitat
The squids are the most active of the cephalopods occurring in coastal waters, in deeper waters, and in the abysses. Their winter habits are little known, but in the spring and in the early summer they swim in large schools of 10 to 100 or more, to lay eggs, when they are netted in great quantities. They can move swiftly both forwards and backwards by the combined actions of the fins and the funnel. When attacked they can emit through the funnel a cloud of black ink to make good their escape. The integument also has a great power of rapid chameleon-like colour changes, which also serves for protection. The skin contains pigment cells filled with various colours. When these cells become larger or smaller, the colours of skin change rapidly as though the animal were blushing. The colour changes harmonise with the colour of background, resulting in concealment of the animal. By examining a young squid under the low power of the microscope, one can observe how this colour ohange is accomplished. Small squids of the deep sea become visible in the dark by means of lurninous organs of various colours. The ink of deep sea liquid is also luminecent as it has to be if it is to serve its purpose in the dark waters of the deep sea.

13.3.2. External Features

Shape and size
The squid has a tapering somewhat torpedo-shaped or spindle-like body, hencethe nickname "sea-arrow".
Unlike other animals, the long axis of the body is dorso-ventral so that when the animal swims, the morphologically ventral surface is anterior, the dorsal surface is posterior, the anterior surface is dorsal and the posterior surface is ventral.
The average length for an adult squid is about 30 cm. but some species are only 25 cm. long.

The largest cephalopods are the giant squids, Architeuthis, of the North Atlantic, one of which attained a total body length of 16 meters. The tentacles alone were 11 meters long and the body circumference 3-6 meters. A giant squid, captured in the Atlantic by the French battleship Alecton, in 1860, was 15 meters long and weighed nearly 2 tons. They possess arms as large as man's legs and suckers as big as tea-cups. in another case, two separate tentacles, each about 12'6 meters long were found. Naturally, they must have come from a much larger specimen.

They have the distinction of being not only the largest molluscs, but also the largest invertebrate animals Since it is limited to the deep sea, it is rarely seen by man and hence is not well known. Most accounts of giant sea- serpents, though never been caught, are probably based on sudden and un expected glimpses of long writhing arms of a giant squid on the surface of the sea The sperm whales feed on these giant squids They said to engage in battles with whales whose dead bodies have been found bearing scars made by the suckers on the arms of a giant squid and their tongues eaten by the sharp beak of the squid.
Body
The body is divisible into an anterior head region and a posterior tapering visceral hump, jointed together by a narrow neck.

Head
The head is distinct, though small, and bears a pair of large eyes and the central mouth surrounded by 10 fleshy arms arranged in a ring. Two powerful horny jaws, like an inverted parrot's beak, protrude from the mouth, the ventral jaw overlapping the dorsal.
The foot is modified into the funnel and the 10 arms surrounding the mouth at the end of the head, 8 arms are short, stumpy and non-retractile and 2 are long, slender retractile tentacles, used in capturing prey. The inner surface of each arm bears pedicellate and cup-like suckers decreasing in size from the base to the tip of the arms. The suckers give the animal a good grip on anything around which it wraps the arms.
Visceral hump
It is long and pointed and bears two conical fins dorso-laterally towards the tail end, by means of which the animal swims forwards.
Mantle
The mantle forms the thick, muscular and protective envelope, enclosing the visceral hump and the mantle cavity. The conical projections of the mantle, one on each side of the animal, form the fins. Ventrally, the free mantle edge forms a loose collar around the neck region, thus leaving a circular opening, through which water enters the mantle cavity. A conical mascular tube projecting beyond the collar, beneath the head, is the siphon or funnel, through which the water of mantle cavity is expelled. The funnel is the true representative of the molluscan foot. The collar articulates with the funnel and the visceral mass by three interlocking surfaces. The visceral mass occupies most of the space in the mantle cavity.

Locomotion
The mantle and the funnel form the chief locomotory organs. The customary mode of the locomotion is slow swimming by the undulating movements of the fins, during which the arms are closely extended in front to serve for steering. But, when the animal is excited, the mantle collar closes tightly around the neck and the water is forcibly ejected through the siphon, so that the animal is propelled in the opposite direction like a rocket or torpedo, after the principal of jet propulsion.
The rocket-like tapering body enables it to dart through water with lightning-like speed. The siphon is the chief steering organ; if it is directed forward, the jet of water passed through it propels the animal backward; if siphon is directed back ward, the animal is propelled forward. Squids attain the greatest swimming speeds among aquatic inver teberates Inspite of its rapid swimming, squid is often caught by large fish and some whales
Shell
The shell is internal, vestigial and embedded under the mantle mid-dorsally, extending from the edge of collar to the posterior end of the trunk. It is thin, light, nearly transparent, and feather-shaped horny plate with a stiffening rib down the centre on one face. It is called the pen or gladius because of its fancied resemblance to an old fashioned besides, a cartilage case surrounds the brain: a nuchal cartilage supports the neck and similiar cartilage support the siphon and the fins. The cartilage is remarkably similar to that of vertebrates and shows an instance of the convergent evolution.

13.3.3. Respiratory System

There are two elonged gills in the mantle cavity, one on either lateral side of the visceral mass to which their proximal ends are attached. By alternate expansion and contraction of the mantle, the respiratory water passes into and out of the mantle cavity through the wide circular opening between the neck and collar. It flows over the gills, supplying oxygen to the blood and carrying away carbon doixide.

13.3.4. Digestive System
Alimentary canal
The mouth leads into a hard and round muscular pharynx or buccal mass, containing a pair of horny jaws and an odontophore with a radula. The jaws resemble an inverted parrot's beak and are moved by strong muscles. The slender oesophageous leads from the buccal mass through the liver to the large, sac-like and thick walled muscular cliverticulum stomach, closely connected with a complex valve in between. The stomach is followed by a short intestine, demarcated by a constriction from the rectum, which terminates in the anus at the base of the siphon. Two anal valves of doubtful function are attached to the sides of the anus.

Digestive glands
The salivary glands, two in the dorsal muscular wall of the pharynx, and one embedded in the ventral end of the liver, open by their ducts into buccal cavity. The liver is a single, median and cone-shaped organ with its broad base near the collar. The pancreas is a small V-shaped structure lying anterior to the stomach. A single hepatopancreatic duct from the liver and pancreas leads into the caccum.
Food and feeding mechanism
The squid is carnivorous, feeding on small fish, Crustacea, Mollusca and other squids. The two long tentacles, each of which is retractile into a pouch of the head, can extended forward to seize the prey, when at a distance, by their suction cups and draw it towards the mouth, where it is held firmly by other arms. The jaws cut large pieces which are swallowed rapidly, the small radula probably seldom used.
Digestion
The food is partially digested in the stomach by digestive fluids or liver and pancreas. In the caecum, digestion is completed and absorption takes place. The indigestible food passes on to the intestine and expelled thr9ugh the anus.

13.3.5. lnk Sac
Like the Octopus and the Cuttle-fish, the squid has an ink-sac for defence. It lies dorsal to the rectum and its duct open into the siphon near the anus.
When in danger, it discharges a black ink which, when spread in water, forms a cloud-like 'smoke screen' which obscures the vision and perhaps numbs the olfactory organs of the enemy, thus giving the squid an opportunity to escape. The black colouring matter probably serves to distract the attention of the enemy, while the squid escapes. Recent investigations have shown that squid does not eject a large cloud cover of ink as has been supposed. But it discharges just enough ink to colour a volume of water of its own size. The predator often mistakes the ink for the squid, which gets a chance to escape. Thus, the squid possesses a defensive device which man has only recently used in warfare.

13.3.6. Blood Vascular System
The circulatory system is well formed and of the closed type. It is unique in that it has two types of hearts. At the base of each gill is a rounded organ, the branchial heart. They receive venous blood of the body through larger veins or venae cavac. Each gill heart pumps blood to the gill of its side. The oxygenated blood from the gills is collected in an auricle on each side. The two auricles join the single ventricle of the systemic heart, which sends arterial blood to all parts of the body by three aortae. In the tissues the arterial capillaries join the venous capillaries, which finally form the larger veins leading to the branchial hearts.

13.3.7. Excretory System
The two kidneys or nephridia are white triangular structures, extending forward from the branchial hearts, and opening by nephridiopores on either side of the intestine at the tips of small papillae.

13.3.8. Nervous System
Brain
The nervous system is very highly developed. Several pairs of ganglia are concentrated in the head, to form a large brain encircling the oesophagus, behind the buccie mass. The two cerebral ganglia, lying above the oesophagus, are fused together forming a single round mass. Below the oesophagus lie two ganglia, an anterior pedal and a posterior visceral. The sides of the oesophageal ring are formed by the paired pleural ganglia, lying lateral to the oesophagus, and giving rise on each side to a large optic nerve, leading to the optic ganglion in the eye. Fibres from the pedal ganglia extend anteriorly a short distance to a pro pedal or brachial ganglion which gives rise to 4 pairs of brachial nerves to the 8 arms and a pair of tentacular nerves to the two tentacles. The cerebral ganglionic mass and the propedal ganglion are connected anteriorly with a small supra-buccal ganglion.

Stellate ganglia

Posteriorly, the visceral ganglion gives off a pair of visceral and a pair of pallial or mantle nerves. The two visceral nerves fuse together immediately after origin but again separate posteriorly. The mantle nerve on each side runs backwards and outwards and sends a branch to a large stellate ganglion, living on the inner dorsal surface of the mantle near the tip of the gill. Several giant nerve fibres radiate into the mantle from the stellate ganglia and are responsible for its rapid contraction when jet propulsion is required. Being several hundred times larger than vertebrate nerve fibres, these have become quite popular with nerve physiologists; because their large size makes it much easier to use them by inserting tiny electrodes for study of the characteristics of the nerve impulse.

13.3.9. Sensory Organs
Eye
The two large eyes are unique in that superficially they resemble those of the vertebrates. However, when critically examined, they are found to be fundamentally different in structure and origin. Each eye has a transparent cornea a pigmented iris, a spherical lens, anterior and posterior chambers and retina with rods, etc. and can form a real image. This resemblance is a case of analogy or convergent evolution.
Statocysts
The two sac-like statocysts, probably organs of equilibrium, lie side by side embedded in the cephalic cartilages of the head, below the visceral ganglion.
Olfactory organ
Immediately behind each eye is a fold, the olfactory crest, the concavity behind which probably acts as an olfactory organ.

13.3.10. Reproductive System
The sexes are separate. There is a single gland lying ventrally in the apical region of the visceral mass, and opening directly into the coelom by a slit. A single gonoduct runs on the left from the coelom to the mantle cavity and opens near the funnel. The male reproductive system includes a testis, a vas deferens, a spermatophoric sac containing sperm packets or spermatophores, and a copulatory organ or penis. The female reproductive system comprises of an ovary, an oviduct, an ovi-ducal gland, and a part of nidamental glands, which produce a jelly in which the eggs are embedded.
During breeding season, the male squid displays a courtship behaviour which was known and described by Aristotle over 2,000 years ago. He transfers the spermatophores by the specialized tip (heterocotylus) of the third right arm to the mantle cavity or within the seminal recepte on the buccal membrane of the female, where the tip of the arm becomes detached and remains free.

The fertilized eggs are deposited in masses, usually found attached to rocks below the tide mark. The eggs are large, with much yolk, and are laid in elongate gelatinous capsules. Owing to the shape of egg masses, they have been known for centuries as "dead man's fingers". Cleavage is meroblastic and the young hatch, within 2-3 weeks, as miniature adults able to swim and feed at once.

13.3.11. Economic Importance of Cephalopods
Cephalopods are of use to man in various ways.
As food
Squids, cuttle-fish and devil-fish are popular articles of the human diet in the Oriental and the Mediterranean countries. In China, Japan, India and Italy, they are sold in the markets for food. Sepia is abundant in the European waters; particularly in the Mediterranean Sea, where large quantities are used as food, either cooked or dried in open air. Small squids like Loligo, sometimes swim in large schools and are netted in great quantities. They are split, sun-dried and preserved for later use. The Red Indians, dwelling along the Pacific Coast in Canada and Alaska, are, said to use the devil-fish (Octopus) on many occasions In the Mediterranean and Oriental countries, South America and the South Pacific, boiled octopus is a much better known and more frequently eaten dish than boiled lobster.

The pearly Nautilus (Nautilus poinpillus) is much prized as food by Pacific islanders.

Cephalopods are also an important source of food for other animals. Squids are gregarious and swim in vast numbers pursued by marine mammals and large fish which feed upon them. The giant squids (Architeuthis) of cold depths of Newfoundland and elsewhere are pelagic, spending their life swimming at various depths and eaten by sperm whales.
As bait
Squids make an excellent bait for marine fishes, especially for cod and are used by tons for this purpose in the United States. The cod, fishermen of the North Atlantic are well aware of the relation between the squids and the cod and express it by the laconic "Plenty squids, plenty cod". Small octopods are captured and used as bait by the line fishermen of Palk Bay, for their flesh is firm, not easily pulled off the hook, and of a tempting odour to fish sought after.
In arts and medicine
A rather odd and unexpected use for fossil cephalopods is found among the Red Indians of Montana and Wyoming. Their medicine men collect specimens of beautifully preserved fossil ammonoids from Cretaceous Strata, and keep them as' medicine". The internal calcareous shell of Sepia is the "cattle-bone" of the commercial world. It is used for various purpose, such as a medicine, a dentifrice, fine polishing agent, lime-supplying food for cage-birds like canaries, an agricultural fertilizer and for taking casts in metal work The cuttle-bones are collected on the Indian seacoasts, during monsoon, when they are drifted ashore in huge numbers.
The contents of the ink-sac of the cuttle-fish provide a rich brown pigment, called "Sepia", used by the artists. The original "India ink" was obtained from the ink of a cuttle-fish, Sepia cul'rara; today a certain brown finish of photographs is termed as sepia finish.
The shell of Nauzilus is also much used in the arts, and for many other useful purposes. It is a pretty object often thrown ashore during monsoon storms on the Indian coasts.
As predators
The cephalopods are all predaceous and carnivorous molluscs, devouring great numbers of fish, crustaceans and other molluscs, and often are very destructive to the fisheries. Crabs are the favourite food of the octopods, but they also feed upon the bivalves, the organic debris of the sea bottom, and occasionally the fish. The giant squids (Architeuthis princeps) of North Atlantic are the largest invertebrate animals with the body length of 15 meters excluding the arms. They are known to engage in battles with whales and devour their tongues by their sharp beaks.
In literature
The giant squids and octopuses have played a somewhat exaggerated role in popular literature. During the days of small sailing vessels, many sailors' tales were current describing the horrors of encounters with them. One such story pictures a huge squid dragging a small ship beneath the waves, after which it would 'grab the helpless sailors in its cruel, snake-like arms and crush them to death. But no authentic report of such an event is known to exist.

Sepia (The Cuttle-fish)

13.4. Cephalopoda - Sepia (The Cuttle-fish)
The cephalopods, whose living representatives include cuttlefish, squid, octopus, nautilus, etc., are marine molluscs very different from the other molluscs in general. Adapted for a swimming existence, they are the most highly organised of all the molluscs and include the largest species of the invertebrate animals. The head region, as the name implies, is large and well developed. A very common cephalopod is the cuttle-fish.
Systematic Position
Phylum- Moliusca
Class- Cephalopoda
Order- Colcoidea (Dibranchia)
Sub-order- Decapoda
Family - Sepiidae
Genus- Sepia

13.4.1. Habits and Habitat
The Cuttle-fish, like the squid (Loligo), is a marine mollusc living usually in shallow coastal waters. It is widely distributed especially in warmer seas like the Mediterranean. It is not a bottom-dweller. It is quite a good swimmer and can swim either forwards or backwards by its fins and funnel. They are found in groups, either swimming freely or\resting on the sea bottom, where it can bury itself. Various species live in different depths of the sea, some as far down as 3,000 \meters. Breeding occurs between late winter and summer when they migrate into deep or shallow water, according to the species. The cuttle-fish can adapt its colouration to its surroundings and, like most other cephalopods exhibit luminescence. It feeds on fish, crabs, shrimps and prawns, etc., and can eject a jet of ink to distract the attention of the, enemy.

13.4.2. External Features
Shape and size
The cuttle-fish has a fishy, bilaterally symmetrical and dorso-ventrally flattened body, which is shorter and flatter than that of Loligo, and tapers towards the posterior end. The anterior and posterior ends of the body in fact represent the dorsal and ventral ends due to much elongation of the dorso-ventral axis of the body. The average size is about 30 cm. The smallest cuttle-fish, belonging to the genus Idiosepius, measures about 15 mm. in length.
The body is divisible into an anterior prominent head and a posterior trunk, united by a constricted neck.
Head
The head bears a pair of large, highly developed eyes at the sides, and a mouth at the free extremity, surrounded by 5 pairs of tapering, muscular circumoral appendages. These are differentiated into 4 pairs of short and stout arms and one pair of long tentacles, retractile into large pits at their base. The tentacles are used in the capture of prey and in copulation. The bases of all but the ventral pair of arms are united by an interbranchial web of integument. The inner flat surface of each arm bears four longitudinal rows of suckers. Each sucker is a muscular, shallow cup with a narrow, horny rim and supported on a short, thick stalk. The suckers can be firmly applied to the body of the prey or to any other object by creating a partial vacuum inside.
In the male cuttle-fish, the left fourth or ventral arm is modified or hectocotyilsed to serve as an intrornitteut organ, by the sup preio fiikers in its basal part. In tentacles, the suckers are found only on their terminal expanded portions. The apex of each tentacle bears a curious small terminal pad.
Trunk
The rest of the body or trunk is elongated and shield-shaped, with its base directed anteriorly and the ahoral nd or the apex posteriorly. It is slightly convex above and flat below and borded by a narrow frill like fin on either lateral side. The fins are separated by a cleft at the aboral end and are used in leisurely locomotion.
Mantle
Trunk is covered by a thick, muscular mantle, enclosing on the ventral side a large mantle cavity, which contains the viscera. Towards the oral end, the free mantle edge, encircling the narrow neck, forms a rounded lobe above and a collar below.
Funnel
Below the head lies a large conical muscular tube, the siphon or the funnel, projecting beyond the neck. It opens externally by a narrow aperture, but internally by a wide aperture, into the mantle cavity. A pair of cartilaginous knobs on the mantle fits into corresponding sockets on the posterior ventral surface of the funnel. A typical molluscan foot is not present. It is represented, in part at least, by the siphonal apparatus. The old concept, that circumoral arms and tentacles correspond to the forefoot or epipodiof the Gastropoda, seems to be erroneous; they are probably true cephalic appendages. Therefore, the name Siphonopóda has been suggested, instead of Cephalopoda, for the class.
Skin
The living cuttle-fish undergoes frequent changes of colour. These are brought about by numerous, large, pigment cells, the chromatophores and iridocytes, which lie in the dermis of the skin. The chromatophores are of three kinds: reddish, yellowish-brown and orange. They can be detailed by the contraction of radiating muscle fibres attached to the cell walls at one end, and to the skin at the other. By alternate contraction and expansion of the chromatophores, blushes of different hues pass rapidly over the surface of the body. The chromatophores, are less on the ventral side, which is much paler than the dorsal. The iridocytes, lying beneath the chromatophores, are transparent cells with a reticulate structure. They defract the light, producing the characteristic iridescence of the skin. The colour changes are controlled by the central nevous system. Preserved specimens usually loose their natural colour.
Shell
The shell of the cuttle-fish is internal, lying embedded, in the upper side, completely enclosed in a sac of the mantle ad secreted by its epithelial lining. It is flat, broad and oval in shape, represented by phragmocone with a broader and rounded oral end, called pro-ostracum, and a narrow, pointed aboral end, called rostrum projecting into a spine.
The shell is entirely dead and composed of calcareous rather than horny matter. The hard and resistant shell provides rigidity to the trunk, like an endoskeleton. The calcareous matter is arranged in fine parallel layers, the septa or laminae, enclosing spaces containing fluid and gas, so that the light shell serves as a hydrostatic organ or float and, owing to its dorsal position, helps in maintaining the equilibrium of the body.

The shell of cuttle fish, or the cuttle-bone, is a familiar object of the seashore. It is rather soft and spongy. Being light, it floats in water, and during monsoon, these bones are drifted ashore in such great numbers that they have been named as the sea-fram. It is given as a bill sharpener as well as a source of calcium to caged birds.

Mantle Cavity
When the mantle is cut mid-ventrally by a longitudinal incision and reflected, the voluminous mantle cavity is exposed. It extends posteriorly up to the apex—of the trunk. The funnel is seen towards the oral end with its narrow external and wide internal Openings. Basally, on its ventral surface, the funnel bears a pair of cartilaginous depressions or sockets, called the called the funnel cartilages. Opposite them, on the internal surface of the rejected mantle folds, are seen a pair of oval cartilaginous knobs, the mantle cartilage. Similarly, the dorsal surface of neck carries the nuchal cartilage, which fits against the dorsal cartilage on the mantle.

The mantle cavity communicates with the outside in two ways, through the wide opening around the neck and by means of the funnel. When the mantle cavity enlarges, the inhalant water is drawn into the mantle cavity by the wide opening between the mantle collar and the neck. When the mantle cavity contracts, an interlocking occurs between the mantle and funnel cartilages ventrally and the nuchal and dorsal cartilages dorsally, so that the collar space is tightly closed. Thus, exhalent water has to pass out through the funnel in the interior of which a Cap-like valve, opening outwards, allows the water to run out from the mantle cavity but not in the reverse direction.

The bulk of mantle cavity is occupied by the visceral dome, consisting of various internal organs. The digestive gland lies anteriorly, bordered on either side by the retractor muscles of head and funnel. The median rectum opens by the central anus at the base of the funnel. On either side of the rectum extends a renal sac, opening into the mantle cavity by an external renal aperture upon a renal papilla. On the left side only is the .genital aperture, also at the end of a papilla. A little posterior lie the two large plume-shaped cienidia, one on each side. Two large stellate ganglia lie one on each side on the mantle wall, where the neck meets the trunk. Posteriorly, the round ink-sac can be recognized by its metallic colour. its duct runs forward ventral to the rectum Sand opens into it dorsally close to the anus. In the male, the testis lies partly covered by the ink-sac. In the female the renal sac is hidden from view by a pair of nidamental glands and a pair of accessory nidamental glands. The genital duct (penis in the male, oviduct in the female) opens on the left side near the left renal aperture.
Locomotion
The cuttle-fish swims about gently by the undulating movements of the fins which are also used in directing the course of the animal. But, the most important movements are the swift, darting movements with the help of the funnel, caused by rhythmical contractions of the mantle. When the mantle relaxes, the mantle cavity is enlarged and water is taken in through the collar space around the neck. When the mantle contracts the mantle-collar tightly fits on the neck closing this opening, so that the water in the mantle cavity is shot violently out through the siphon like a jet, and animal is propelled rapidly through water in the opposite direction. The resulting backward movement is so sudden and rapid that the cuttle-fish seems to-vanish instantly. The siphon may also determine the course of direction; when it is pointed forward, the cuttle-fish darts backward, and when, it is pointed backward, the animal is driven forward. The quick reaction is made possible by giant fibres in nerves, over which impulses travel very rapidly as in giant fibres of the earthworm. Man has employed jet propulsion in rockets and jet aircraft.
Coelom
The coelom is represented by the viscero-pericardial coelorn and the cavities of renal sacs. The former is a larger bag-like cavity extending backwards and divided by a constriction into two parts. The anterior part or pericardium encloses the hearts and communicates by two reno-pericardial apertures with the cavities of renal sacs. The posterior part or gonocoel encloses the gonad.

13.4.3. Digestive System
Alimentary canal
The mouth, lying in the midst of the oral arms, is surrounded by a fleshy, circular lip, beset with numerous papillae. Just within the lip is a pair of sharp, powerful, horny jaws, looking like the inverted beak of a parrot. The mouth leads into a large thick-walled, muscular pharynx or buccal cavity, containing tongue or odontophore and the raaula. The oesophagus is a long narrow tube running straight backwards between the two lobes of the liver to open into a rounded thick-walled muscular bag, the stomach. A large thin walled and slightly- coiled pouch, the caecum, is connected to the stomach close to the starting point of the intestine. The short intestine runs anteriorly nearly parallel with the oesophagus and merges into the rectum which opens into the mantle cavity by the anus. A pair' of leaf-like anal valves, of uncertain function, project at the sides of the anus.

Digestive glands

Sublingual glandular tissue of unknown function lies on the ventral side of the tongue. A pair of anterior salivary glands lies within the buccal mass opening on either side of the radula. Pair’s of posterior salivary glands, lying in front of the liver, one on either side of the oesophagus, open by a common duct at the tip of the tongue in buccal cavity. The salivary glands have been misnamed because they are an really poison glands and their secretion is used to paralyse the prey. The large, brown digestive gland or liver extends from near the posterior end of the body. It is a solid, btobed gland, giving off one duct from each lobe. The two ducts pass through Ink-duct' the dorsal renal chamber and unite to open into the vestibule or the chamber where the stomach, the caecum sac and the intestine meet. The two hepatopancreatic ducts bear minute vesicles which constitute the pancreas.
Ink gland
As already stated, a pear-shaped ink sac lies over the posterior ventral surface of visceral dome and opens by a duct dorsally into the rectum close to the anus. The terminal part of the duct forms on ejecting ampulla. The ink gland, lying inside the wall of the large reservoir or ink sac, secretes a brown or black fluid or ink. It contains a high concentration of melanin pigment and is stored in the ink sac. When the cuttlefish is startled, it discharges the ink through the funnel as a black cloud, which forms a sort of smoke-scren or a dummy, under the cover of which the animal escapes from an enemy or approaches a prey. The ink of Sepia provides sepia pigment used by artists for hundreds of years.

Food and feeding mechanism

All cephalopods are carnivorus. The food consists chiefly of Crustacea, Mollusca and small fish. The tentacles are rapidly extended and attached to the living prey by the suckers. Then the tentacles retract, so that the food is brought within the reach of the arms which hold it. The prey is paralysed by the poisonous secretion of the salivary glands, broken into pieces by the jaws and swallowed probably by the aid of the radula.
Digestion
Inside the stomach, the food mixes with the fluids from the liver and pancrease. The semi-digested food passes to the spiral caecum, where digetion is completed. The liquid products of digestion are absorbed in the caccum, while undigested food is passed on to the intestine where some absorption may take place.- The residual food is expelled out of the anus. -
The hepatoancreas not only secretes digestive enzymes but also serves for the absorption and storage of food and the excretion of waste products. The cephalopods differ from the majority of invertebrates in that probably the food stored in the liver is not absorbed directly but received through the blood by the liver.

13.4.4. Respiratory System
Respiratory organs
These include a pair of large, plume- shaped cienidia, lying in the mantle cavity, one on either lateral side. Each ctenidium or gill is bipinnate, with nunierous delicate lamellae on either side of a central axis. The surface of the lamellae is much folded to increase the respiratory surface. Cilia are absent as removal of sediment is not a problem in pelagic animals and water current is created by mantle contractions Each gill receives venous blood through an afferent branchial vessel from the branchial heart of its side. Inside the gill it passes through a system of minute branches through the lamellae and is collected finally into an efferent branchial vessel leading to the auricle.
Respiratory mechanism
The muscular mantle rhythmically expands and contracts, so that the mantle cavity alternately increases and diminishes in size. Consquently, the oxygen bearing inhalent current of water enters the mantle cavity through the wide aperture around the neck, and the exhalent current escapes through the funnel. Exchange of gases occurs when the water passes over the ctenidia which are richly vascular.

13.4.5. Circulatory System
The blood vascular system is well developed with a complete separation of venous and arterial blood. It consists of the heart, arteries, veins, and a system of capillaries.
There are three hearts in Sepia, as in all the dibranchiate cephalopods. The systemic or arterial heart lies in the middle of the visceral mass enclosed in the pericardium. It consists of a thick-walled median ventricle, and two thin walled lateral auricles, all spindle-shaped. The ventricle is slightly constricted into two lobes; it supplies arterial blood through a large anterior or cephalic or oral aorta and a small posterior or aboral aorta, to the anterior and posterior regions of the body, respectively. The aortae branch into arteries which lead into a system of capillaries and then into veins.
The venous blood of the head returns by a large, vena cava, which bifurcates in front of the rectum into right and left branch ial vein leading into the so-called branchial harts, laying one at the base of each ctenidium. Each banchial or gill, heart also receives directly a small pallial vein from the mantle and an abdominal abdominal vein from the posterior body region. The unpaired genital and ink sac veins join the right branchial vein. Each branchial heart pumps blood to the ctenidium of its side through an afferent branch/al vein, running through the axis of the gill and giving off branches as it goes. The oxygenated blood of a ctenidium is returned by an efferent branchial vein first to the auricle of that side, and then to the ventricle.
The blood, containing haemocyanin and amoehocytes, is colourless when venous and pale-blue when oxygenated.

13.4.6. Excretory System
It includes a kidney or renal sac consisting of three thin-walled chambers, two ventral and one mid-dorsal, which communicate with one another. The two ventral chambers open, at one end, to the exterior by renal apertures placed on renal papillae, lying one on either side of the rectum, and at the other, communicate with the pericardium by reno-pericardial apertures. Through each ventral chamber passes the corresponding branchial vein, formed by the bifurcation of the vena cava. The vein is covered by excretory glandular epithelium which extracts the nitrogenous waste products from the blood. The dorsal chamber encloses the pancreatic follicles covering and opening into the ducts of the digestive gland. They are richly vascular and are said to serve an excretory function. The nitrogenous excretory substance has been detected in the cavities of the renal sac in the form of guanin which is discharged into the mantle cavity.

13.4.7. Nervous System
The nervous system of Sepia, as of all the cephalopods, shows a high grade of organization, attained only by some insects and arachnids among the other invertebrates.
Brain
The brain consists of four typical molluscan ganglionic masses, all concentrated in the head, round the oesophagus, behind the buccal mass, and protected by a cartilaginous "skull". A pair of cerebral or supra-oesophageal ganglia are fused together into a rounded mass,'lying dorsal to the oesophagus. Laterally, they give off a pair of extremely stout optic nerves which at once expand into large kidney shaped optic ganglia of the eyes. A small olfactory gang/ion lies on the dorsal side of each optic nerve. Anteriorly, a pair of slender cerebro-buccal connectives connects the cerebral ganglia to a pair of superior buccal ganglia, which are situated dorsal to the buccal mass, and connected by circurnoesopha geal connectives to a pair of inferior buccal ganglia lying below the buccal mass.

A pair of stout circum-oesophageal connectives connect the certhral ganglia to rest of the brain lying ventral to the oesophagus. The suboesophageat ganglionic mass, lying beneath the

oesophagus, is partly divided into an anterior lobe, the brachial ganglion, and a posterior lobe, the pedal ganglion. From the brachial ganglion run forward ten brachial nerves to the arms; this is one of the reasons to regard these arms as portions of the foot. It is also connected to the superior buccal ganglia by paired brachio-buccal connectives and to the cerebral ganglia by cerebro-brachial connectives. The pedal ganglion supplies the funnel.
A pair of pleuro-visceral ganglia is also united to form a single mass lying in contact with the pedal, behind the oesophagus. They give off two pairs of main nerves, directed posteriorly. The visceral nerves, supplying the various internal organs, form a visceral loop from which springs a pair of branchial nerves, innervating the gills and bearing each a branchial ganglion at the base of the gill. The stout pallial nerves innervate the mantle. To bring about rapid and synchronous muscular movements of the arms, siphon and mantle, there is present a system of giant motor neurons, with its centre lying in a median ventral lobe of the fused visceral ganglia.
Stellate ganglia
The palial nerve on either side runs backwards through the neck to the inner surface of the mantle cavity, were it divides into two branches. The outer branch immediately terminates into a large, roughly triangular, stellate ganglion which can be seen without dissection, in front of the ctenidium when the mantle cavity is opened. Several nerves, arising from its outer border, innervate the mantle. The inner branch is connected to the stellate ganglion by two commissures, and innervates the fin.
Sympathetic system
A pair of sympathetic nerves, arising from the inferior buccal ganglion, runs posteriorly along the oesophagus to join gastric ganglion lying between the stomach and the caecum. The gastric ganglion sends nerves to the liver stomach, caecum and intestine.
Sense Organs
The special sense organs are very well developed and comprise paired eyes, statocysts, ciliated olfactory pits and an unpaired gustatory organ.
Eyes
The paired eyes are large, efficient and bulge from the dorso-lateral sides of the head. They bear striking resemblance to those of a vertebrate in that a cornea, iris, lens and retina are present. The lens projects an inverted image on the retina, as in the vertebrate eye. External muscle attachments enable limited movements of the eye. But the embryological development of the cephalopod eye is entirely different from that of the vertebrate eye, so that homologically they are different, for the vertebrate eye is formed as an outgrowth of the brain, while the cephalopod eye is formed by an ectodermal invagination. The similarity between the two is due to convergent evolution, that is, similarity which is not due to phylogeny.
Each eye lies within an orbit formed by cartilages. The outer wall of the eyeball, or the sclerotic coat, is strengthened by cartilage, and covered by a silvery membrane. It extends in front as the contractile iris presenting a large central opening, the pupil, which can be increased or diminished by muscles. Just internal to the iris lies a large, almost spherical lens, consisting of two piano-convex halves, and held in place by a ciliary body. A choroid is absent. The inner sensitive layer, or retina, is somewhat complicated in structure and is composed of a layer of parallel rods, there being no cones. Close behind the eyeball, the optic nerve swells up into the optic ganglion, from which several bundles of nerve fibres are distributed on the posterior surface of the retina. A small optic gland or white body of unknown function lies near the optic ganglion. A true cornea is also lacking. The transparent horny portion of the skin, covering the exposed surface of the eye, is termed as the false cornea. The skin also forms protecting lids. The cavity of the eye is divided by the lens into a small anterior chamber filled with a water-like aqueous humour, and a large posterior chamber containing a jelly-like vitreous humour. The cephalopod eye can accommodate itself to light changes both by modification in the pupil's size and by the migration of pigment in the retina. It can probably an etect colour.

Statocyts

A pair of statocysrs, which are organs of equilibrium or balance, lies ventral to the pleuro-visceral ganglion, enclosed in the cranial cartilage. Each statocyst is a small spherical body containing a large statolith and a fluid. The inner surface, lined with a flattened epithelium, is raised into numerous processes, so that the cavity of the statocyst has a very irregular shape.
Olfactory pits
The osphradia of the usual type are lacking. Instead, a small ciliated olfactory pit is situated posterior to each eye. The sensory cells of the pit are innervated from the small olfactory ganglion lying close to the optic ganglion.
Gustatory organ
On the floor of the buccal cavity, just in front of the odontophore, is a small elevation covered with papillae. It is said to be the organ of taste.
Finally, tactile or otherwise sensitive cells are also found on tharms, tentacles and elsewhere.

13.4.8. Reproductive System
The sexes are separate. The males are usually smaller; less rounded dorsally, and possess slightly longer arms.
Male reproductive system
The large, oval, yellowish and saccular testis lies near the apex of- the visceral mass. Sperms produced in the testis are passed into its lumen which opens into small ciliated aperture on the left side into a long sperm duct or vas deferens, which opens into a long seminal vesicle. Here, the sperms are rolled up into long and narrow bundles enclosed in elaborate chitinous capsules, called the spermatophores.

A spermatophyte like an automatically explosive bomb; at one end et has a complex spring-like arrangement which ruptures its wall and discharged the sperms after copulation. The terminal part of seminal vesicle gives on two blind folds, one of them being the prostate or accessory gland. The seminal vesicle terminates into a wide reservoir, the sperm sac or Needham's sac, which opens into the mantle cavity by the genital aperture lying on a papilla or the pent to the left of the anus.

Female reproductive system
The largo rounded white and saccular ovary is also situated, like the tests m the male, in a chromic sac near the apex of the visceral mass. The oviduct, leaden: from the codomlc sac is a short, thin-walled, wide tube which opens into the mantle cavity to the left of the anus. The narrower distal end of the ovidud has truck glandular wails forming the oylducal gland, the function of which is to secrete the outer coat of the ova. A pair of large oval and flattened nidamental glands lies one on either side of the ink duct each opens by its dad anteriorly into the mantle cavity. A pair of orange coloured accessory nidamental glands is situated in front of the nidamental glands, opening into the mantle cavity by numerous minute pores. All these glands serve to secrete the elastic egg Capsules.
Copulation
In Septa, the fourth left arm of the male is specially modified as an intromittent organ for copulation by the suppression of some basal rows of suckers, and is called the hectocotylus. The male inserts it in the own mantle cavity to extract the spermatophores, or the elastic capsules felled with sperms. Next this arm is thrust into the mantle cavity of the female with its load of spermatophores which are quilted on the bursa copulatrix, or a modified part of the funnel.
The eggs are larger and the developing embryos feed on large amount of stored food yolk. The young ones that hatch out are like the adults.

Part II-Biology

1.    Food and Feeding Habits of Shellfishes

1.1.                        Penaeid shrimp

Penaeid shrimps are mostly omnivorous, feeding at the muddy bottom. Their post-larvae and juveniles feed on detritus but sub-adult prawns prefer polychaetes, bivalves, gastropods, benthic copepods, ostracods, amphipods and foraminifers. The adults of larger penaeids become predaceous and feed on cephalopods and smaller species of prawns and fishes. e.g. Fenneropenaeus indicus (Indian white prawn), P. semisulcatus (Green tiger prawn), P. monodon (Giant tiger prawn), P. merguiensis (Banana prawn), P. japonicus (Kuruma prawn), P. penicillatus (Red-tail prawn), Metapenaeus dobsoni (Flower-tail prawn), M. monoceros (speckled prawn), M. affinis (Jinga prawn), M. kutchensis (Ginger shrimp), M. brevicornis (Yellow prawn), Parapenaeopsis stylifera (Kiddi prawn), P. hardwickii (Spear prawn), P. sculptilis (Rainbow prawn), P. maxillipedo (Torpedo prawn), P. uncta (uncta prawn), Tranchypenaeus curvirostris (Rough prawn), Metapenaeopsis stridulans (Fiddler shrimp), Parapenaeus longipes (Flaming prawn), Solenocera crassicornis (Coastal mud prawn) andS. choprai (Coastal mud prawn).

1.2.  Non-penaeid shrimps 

Non-penaeid shrimps mainly feeds on detritus consisting of fibrous and granular material of phyto and zooplankton origin. Nematopalaemon tenuipes feeds mainly on the planktonic crustacean. Exhippolysmata ensirostris is highly predaceous and feeds on Acetes, polychaetes and young ones of fish and shrimps. e.g. Acetes indicus (jawala paste shrimp),Nematopalaemon tenuipes (spider prawn) and Exhippolysmata ensirostris (Hunter shrimp).

1.3. Crabs

Crabs feed mainly on smaller crustaceans, fishes, molluscs, polychaetes, detritus, bits of plant and other organic materials. e.g. Portunus sanguinolentus, P. pelagicus, Charybdis feriatus, C. annulata and C. natator.

1.4. Lobster

Lobsters generally prefer mussel and clam. Occasionally, they eat smaller crustaceans, polychaetes, fishes while scavenging. e.g. Panulirus polyphagus, P. homarus, P. ornatus, P. longipes, P. versicolor and Thenus orientalis

1.5. Cephalopods

The cephalopods are generally carnivorous and their food consists of teleost fishes, crustaceans and other cephalopods. Cannibalism is common among cephalopods. Feeding intensity decreases during the spawning season. e.g. Sepidae (true cuttle fishes), Sepiadaridae (bottle tail squid) and Sepiolidae (bobtail squid).

2.    Larval Forms in mollusca

In molluscs, the development may be direct or indirect. In the direct development, there will be no larval stage and the young ones will hatch out from the eggs which resemble their parents in general appearance, except the size. The direct development is seen in cephalopods. In indirect development, a larval stage is seen in majority of molluscan forms. Paludina is a viviparous form. Three main larval forms are observed in molluscan forms. There are:

1.   Trochophore

2.   Veliger

3.   Glochidium

2.1. Trocophore

The ciliated trochophore in its typical state appears in Palella and Trochus and in these it is formed at a very early period, even before the formation of the mesoderm, and becomes free at once. It is pear shaped and measures about 0.5 mm in length. A circle of preoral cilia, the prototroch or velum divides the body into two unequal parts, the upper one consist of prostomium whereas the lower part bearing mouth and anus. The preoral part is large and convex with an apical plate bearing the long cilia called the apical cilia as seen in the larvae of patella, dentalium etc.

Near the apical cilia, there are two ciliated elevations each consisting of a single cell. At the posterior end, there are large anal cells bearing a bunch of cilia called telotroch. Between the ciliary girdle and anus, on the dorsal side, the area of ectoderm constitutes the shell glands which secretes a thin, saucer shaped larval shell. The trochophores of lamellidae consist of an additional shell inside the first one which will be cast off later. A slight ridge in the neighboring area of shell represents the border of mantle. Alimentary canal is made up of single layered epithelial cells in the form of a tube. It comprises mouth, stomodaeum, oesophagus, stomach and intestine (mesenteron). A single large cell gives rise to the mesoderm. A rudiment of radula is present in oesophagus. The trochophore contains two cerebral ganglia, a pair of eyes from the underside of apical plate. The statiolith sacs appear as depression of ectoderm at the sides of the mouth. Soon the trochophore develops into a yet more efficient locomotor form the veliger.

2.2. Veliger

The preoral ciliate area or velum begin to protrude on both sides as a bilobed flap. The velum are very delicate and extensive, and a very delicate organs of locomotion. Between the bases of velar lobes, the anterior end of the larva is provided with eyes and tentacles. The larva has a shell into which the velar apparatus could be withdrawn and which encloses the partly developed viscera. Alimentary canal is complete and the anus is shifted to anterior side. A foot usually bearing an operculum if present. In addition to velar apparatus, there are some organs like larval heart and kidney also present. The larval kidneys are paired and generally symmetrical organs situated at the anterior end of the body immediately behind the velum. Statocyst and gill-rudiments are also present. Some gastropods have feeding veligers with a larval life that may last as long as three months. Others have brief yolk-laden nonfeeding veliger. The long cilia of the velum function not only in locomotion but also in suspension feeding. During the course of the veliger stage, torsion occurs and the shell and visceral mass twist 180^o in relation to the head and foot. 

2.3. Glochidium

The glochidium larva is enclosed by two valves, each edge of which bears a hook. The shell valves cover a larval mantle, which bears four groups of sensory bristles. A rudimentary foot is present, to which is attached a long adhesive thread, the byssal thread. There is neither mouth nor anus and which measures from 0.1 mm to 0.5mm depending upon the species.

The glochidia of anodonta, immediately clamp into the fins and other parts of the body surface of the fish. The glochidia require certain species of fish as host; others can tolerate a wide range of host species and the byssal thread aids in initial contact and adhesion (Wood,1974). The larval mantle contains phagocytic cells that feed on the tissue of host and obtain nutrition for the developing clam. The parasitic period lasts from 10-30 days. In the mean time, the parasite is surrounded by the over growth of skin of fish forming a cyst. Some of the larger freshwater mussels may produce as many as 3,000,000 glochidia.

3.Developmental Stages in Cephalopoda

The egg is usually very large and contains a great quantity of nutritive yolk, like sharks, reptiles, and birds. It belongs to the telolecithal meroblastic type, and is enclosed in a capsule. A number of such capsules may become cemented together to form strings. The partial segmentation takes place at the animal pole of the egg, and leads to the formation of a germinal disc (blastoderm).

3.1. Ontogeny of Sepia

One blastoderm grows very slowly round the yolk so that after long time all the outer organs of the embryo are quite recognisable in the region of the original germinal disc; the opposite pole is still occupied by the yolk. Two germinal layers lies in such a way that the centre of the germinal disc or animal pole is placed dorsally, and corresponds with the uppermost point of the visceral dome of the adult animal, while the mass of nutritive yolk lies ventrally.

3.2. 1^st stage

In the centre of the germinal disc there is an oval-rhombic bulging which is the rudiment of the visceral dome and the mantle. On each side, a bean-shaped prominence, the rudiment of the eye appears. Behind the eye, on each side, a long narrow ridge runs backward in a curve; about half way down. This ridge is a small prominence, the rudiment of the funnel cartilage, forms close to its outer side. The part of the ridge lying in front of this prominence becomes the muscle which runs from the funnel to the nuchal cartilage; the posterior part (which lies behind the rudiment of the visceral dome and mantle) forms the paired rudiment of the funnel itself. Between the two rudiments of the funnel, two other prominences rise symmetrically which are the rudiments of the gills. A pit in the centre of the rudiment of the visceral dome indicates the rudiment of a shell gland.

3.3. 2^nd stage

The rudiments just described earlier becomes more prominently. On the outer and posterior sides of the rudiments of the funnel, the rudiments of the two posterior pairs of arms first appear, then those of the third and fourth pairs. The first indications of the head are seen in the form of a large double swelling on each side as the outer and anterior part of which carries on each side as the rudiment of the eye. The embryo becomes covered with cilia. At the extreme anterior end the mouth appears in the middle line, forming the opening of the oesophagus, which begins to sink inwards.

3.4. 3^rd stage

The whole embryo has become more arched dorsally, and more marked off from the yolk. On the latter, the blastoderm, which consists of two layers, an external ectoderm and an internal yolk membrane, has spread out further towards the ventral (vegetative) pole of the egg. At the posterior edge of the rudiment of the visceral dome, the mantle fold has grown out forward in such a way as to form a small mantle cavity, which already partly covers the rudiments of the gills: In the space between the rudiments of the funnel and the gills, theproctodaeum has formed by invagination, and its aperture, the anus could be seen. The rudiment of the fifth pair of arms appears.

3.5. 4^th stage

The visceral dome projects further, and have a free mantle edge all round its base. The gills have shifted further into the mantle cavity, which is now larger, and lies posteriorly. The rudiments of the funnel also now lie close to the mantle. The rudiments of the arms have shifted from behind further forward round the rudiments of the head. As the whole embryo projects more distinctly from the yolk, the rudiments of the arms shift near to one another and under the rudiments of the head. The anus is already covered by the mantle fold.

3.6. 5^th stage

The arms shift still nearer to one another (i.e. towards the axis of the embryo), grouping below the rudiments of the head (which have become fused), and form a somewhat narrow circle on the ventral side in such a way that, when the embryo is seen from the dorsal side, some of them are hidden by the head. As a consequence of this, the embryo, which is already recognizable as young Sepia, now becomes sharply constricted from the yolk beneath it. The free edges of the rudiments of the funnel fuse and move to a position within the mantle cavity.

3.7. 6^th stage

The rudiments of the head and arms have now assumed the typical position to form the "head". The embryo is now altogether distinct from the yolk, to which it merely hangs instead of, as before, lying upon it. The blastoderm finally grows round the yolk and forms a yolk sac. At first this sac is four or five times the size of the embryo, but in proportion as the latter grows at the expense of the yolk and develops further, the sac becomes smaller, so that when the embryo is hatched out, the size of the yolk-sac is only one-third of the young animal. 

It must further be mentioned with regard to the yolk sac that it is at no time in communication with the intestine. As the embryo becomes constricted from the yolk, the latter divides into two parts-an inner part, lying inside the embryo and an outer part, filling the sac. These two parts are connected by means of the stalk of the yolk sac, which projects downward from the "head". The yolk within the embryo is divided into three unequal parts, the largest of which fills the visceral dome; another mass of considerable size fills the "head", and these two masses are connected with a smaller portion lying in the nuchal region.

4.Early development in shrimps

4.1. Eggs

Eggs opaque, with a narrow perivitelline space, chorion has a purplish sheen, diameter of eggs varied from 0.25 to 0.27 mm and that of yolk mass 0, 22 to 0.24 mm. The eggs when first observed at 23.45 hours were covered with a, radiating jelly like substance which partly dissolved and became granular while being observed under the microscope, and disappeared after 3 minutes. The egg was then spherical but without perivitelline space and appeared to be still invested with jelly like substance which was transparent. A polar body was seen adhering to the surface of the egg. Within one minute the perivitelline space was formed by the elevation of the fertilization membrane and the egg assumed the definitive form. Immediately after a second polar body was seen coming out of the yolk mass and traversing the perivitelline space just below the first polar body and soon reached the surface of the egg. The first cleavage began at 00.15 hours, about 30 minutes after the extrusion of the eggs. The second cleavage took place at 00.30 hours. The cleavage continued and at 01.30 hours the blastula stage was observed. An embryonic membrane was clearly visible during the blastula stage. Gastrulation started at 02.15 hours and continued up to 02.55 hours. At 04.50 hours the embryonal mass became constricted laterally and the appendages started differentiating. By 07.45 hours all the 3 naupliar appendages could be seen as lateral thickenings which became tipped with short spine-like setae by 09.45 hours. At 13.00 hours the 3 appendages were fully formed with long setae. The embryo occupied the entire space inside the egg and the movements were restricted to sudden jerks of appendages. The furcal setae first pierced the egg membrane and the nauplius wriggled out of the egg 16 to 17 hours after the eggs were spawned. 

Nauplius I

MTL: 0.30 mm (0.28 - 0.31 mm); MW: 0.17 mm (0.15-0.17 mm); MFS: 0.13 mm (0.11-0.14 mm). An ocellus present at anterior median region of body, dorsal surface of body bears posteriorly a small median denticle a pair of dorsally curved caudal setae present at posterior end of body, 3 pairs of appendages present; A1 uniramous, with 2 long setae of almost same length and a small rudimentary spine-like seta at its apex, 2 short setae on inner distal margin and one long seta on outer distal margin; A2 biramous, endopod shorter than exopod, bearing 2 long setae and one rudimentary seta at apex, and 2 short setae along inner margin, exopod carries 5 long setae along inner margin and tip; Md biramous, shorter than other appendages, bearing 3 long setae on endopod and exopod; setae of appendages nonplumose. Duration of this substage was 4to 4 hours.

Nauplius II

MTL: 0.31 mm (0.29-0.32 mm); MW: 0.17mm (0.15-0.18 mm); MFS: 0.14 mm (0.13-0.15mm)

Setae on appendages plumose; no change in number of setae on A1, but outer terminal and outer lateral setae distinctly smaller than in Nauplius I, inner distal rudimentary seta of nauplius I transformed into a short seta; exopod of A2 with an additional rudimentary seta on outer distal margin, the 4th seta counting from the proximal end bifurcates this bifurcate condition is retained in later naupliar substages; Md comparatively longer; furcal setae show a faint demarcation at proximal 1/3; duration of this sub stage is 3 to 4 hours.

Nauplius III

MIL: 0.31 mm (0.29-0,32mm); MW: 0.16 mm (0.14-0.17 mm); MFS: 0.14mm (0.13-0.15 mm).

No appreciable increase in body measurements; furcal lobes each with 3 setae of which innermost very small and slightly ventrally placed and hence not clearly visible in dorsal view; no increase in number of setae on appendages; among A1 setae inner terminal seta longer and outer terminal seta shorter than in nauplius II; rudimentary setae at tip of A2 exopod and endopod in nauplius II has become longer and plumose. Duration of this sub stage is 6 to 8 hours.

Nauplis IV

MTL: 0.36 mm (0.34-0.38 mm); MW: 0.17 mm (0, 15-0, 18 mm); MFS: 0.20 mm (0.19-0.21 mm).

The furcal lobes become more distinct and bear 4 setae each, outermost seta smallest and being dorsally placed not clearly visible in ventral view; rudiments of developing Mx1, Mx2, Mxp1, Mxp3 seen inside cuticle; A1 outer lateral seta lost and one very small seta added on inner lateral aspect proximally; proximal portion with indistinct segmentation; exopod of A2 with 6 long plumose setae and one rudimentary spine-like seta distally, indistinct segmentation seen in exopod, inner terminal seta on A2 endopod longer. Duration of this sub stage is 3 to 4 hours. 

Nauplius V

MTL: 0.38 mm (0.35-0.41 mm); MW: 0.17 mm (0.15-0.20 mm); MFS; 0.23 mm (0.20-0.28 mm).

Furcal lobes well developed, each carrying 6 setae minute outermost one being dorsally placed; rudimentary oral appendages become bigamous; endopod of A2 with 2 short setae on inner lateral margin and 3 long plumose setae and 1 rudimentary seta terminally, exopod with 9 setae along inner and distal margin, of which distal outer and inner proximal rudimentary and spine like; a prominent rounded swelling appears at base of Md; no change in A1 setation. Duration of this sub stage was 10 to 12 hours. 

Nauplius VI

MTL: 0.48 mm (0.43-0.54 mm); MW; 0.20 mm (0.18-0.21 mm); MFS: 0.31 mm (0.29-0.34 mm).

Body more elongated, frontal organ and carapace clearly demarcated, append ages not clearly segmented, but surface with annular indentations, furcal lobes with 7 setae each one minute and 2 short setae added to A1 on distolateral aspect, A1 proximally with 5 indistinct segments; endopod of A2 with 3 long setae and 1 short one terminally and with a rudimentary seta added to root of distal seta on inner lateral margin, exopod with 10 setae along inner and distal margin, of which newly added distal outer one rudimentary. Duration of this sub stage was 15 to 24 hours. 

Protozoea I

MTL: 0.88 mm (0.88-0.91 mm); MCL: 0.42 mm

Carapace anteriorly rounded, with median notch, frontal organs present as rounded protuberances, ocellus of nauplius persists, developing compound eyes covered with carapace, body divisible into 3 parts, carapace covered anterior region, 6 segmented thorax in middle and posterior unsegmented abdomen; newly hatched protozoea with a swelling in anterior part of the abdomen which is replaced in advanced protozoea I by 5 somites resulting in lengthening of abdomen last abdominal somite and telson not separated by a movable joint, each lobe of caudal furca with 7 setae, outermost seta originates from dorsolateral aspect of furca and is dorsally disposed.

A1 3 segmented, basal segment with 5 sub segments, middle segment with 3 setae and distal segment with 2 setae of which one is long, about twice length of A1 peduncle and 2 aesthaetes, a spike-like setal rudiment present on distal inner margin of terminal segment; A2 biramous, endopod 2 segmented and exopod 10 segmented, 1st segment of endopod with 4 plumose setae of which 2 are placed together near inner distal margin of 1st joint, distal segment with 5 plumose setae of which inner one is smallest, exopod with 11 plumoce setae along inner and distal margin and 2 small setae on outer margin; Md flattened, without exopod and endopod, incisor process with 2 or 3 teeth and molar with transverse rows of smaller grinding teeth 1 free standing tooth present between molar and incisor process; Mx1 with unsegmented protopod having 2 lobes, proximal with 7 and distal with 4 setae, some setae stout and distally spinose, exopod small, knob like with 4 long feathery setae, endopod 3 segmented, distal segment carries 5 long plumose setae, basal and middle segments carry 3 and 2plumose setae respectively; Mx2 with protopod having 5 lobes on inner margin, 1st lobe with 7 or 8 setae, 2nd and 3rd with 4 setae and 4th and 5th with 3 setae respectively, exopod knob-like, with 5 long feathery setae, endopod 4 segmented, terminal segment with 3 long setae distally, the other 3 segments each with 2 long setae on inner margin; Mxp1 biramous, longer than Mx2, protopod 2 jointed, coxa with 4 to 5 and basis with 12 setae along inner margin, exopod unsegmented, carrying 7 plumose setae, 4 along outer margin, 2 terminal and one sub terminal on inner margin, endopod 4 segmented, 1st, 2nd, 3rd and 4th segments carry 3, 1, 2 and 5 long plumose setae respectively; Mxp2 shorter than Mxp1, protopod 2 segmented, coxa with 2 and basis with 5 setae along inner margin, exopod unsegmented, with 6 plumose setae; 3 along outer margin, 2 terminal and one sub terminal on inner margin, endopod 4 segmented, carrying 2, 1, 2, and 5 plumose setae on segments 1, 2, 3 and 4 respectively. Duration of this sub stage was 24 to 48 hours.

Protozoea II

MTL: 1.52 mm (1.40-1.55 mm); MCL: 0.74 mm (0.71-0.76 mm).

Presence of a well developed curved rostrum, bifurcated supraorbital spines, stalked compound eyes and absence of frontal organs distinguish this sub stage from the previous one.

A1 with distal segment bearing 4 aesthaetes and 2 long setae; Md asymmetrical, right and left Md with 1 and 5 free standing teeth between incisor and molar processes; Mx1 with 8 setae on distal lobe of protopod; Mxp1 with 2 plumose setae on 2nd segment of endopod.

As in the previous stage, the larvae show increase in length towards end of this stage. There is a definite increase in body length, MTL being 1.88 mm (1.72-1.99 mm) and MCL: 0 83 mm (0.78-0 84 mm). Advanced stage of protozoea II can be easily distinguished from the early stage by the presence of developing buds of five pereopods and Mxp3 and by the increase in length of abdominal segments. Moreover, the developing uropods can be clearly seen inside the lobes of caudal furca. Duration of this sub stage was 48 to 72 hours. 

Protozoea III

MTL: 2.69mm (2.41-2.73 mm); MCL: 1.01 mm (0.98-1.05 mm).

Supraorbital spines not bifurcate, telson demarcated from 6th abdominal segment by an articulating joint, abdominal segment 1 to 5 with dorsomedian spine on posterior border, 5^thand 6th abdominal segments have each a pair of posterolateral spines 6th segment devoid of posteromedian dorsal spine, but with a pair of ventrolateral spines, caudal furcae bear 8 setae each, a pair of biramous uropods present, exopod of uropod slightly longer than endopod and bears 6 terminal setae, endopod has 2 terminal setae, buds of pereopod and Mxp3 well developed and biramous, exopod bud of Mxp3 with 3 terminal setae in advanced larvae of this sub stage, exopod and endopod of uropod are almost of same size and uropod rami reach much beyond middle of telson an increase in length of biramous buds of thoracic legs is also noticed.

A1 3 segmented, sub segments of basal segment fused into one, basal segment with one distal seta, 2nd segment with 2 lateral setae and 3 to 4 setules, distal segment with 3 or 4 aesthaetes and 3 setae of which one is long being more than twice length of peduncle, distal segment appears to be the forerunner of outer A1 flagellum; Md asymmetrical, between incisor and molar processes there are 6 free standing teeth in left Md and 2 free standing teeth in right Md; Mx1 with 10 setae on distal endite of protopod while setation on proximal endite remains unchanged; Mx2 with more setae on protopod endites, exopod and endopod remaining unchanged; Mxp1 with 8 setae on coxa and 12 setae on basis of protopod and 9 setae on exopod; Mxp2 with 7 setae on exopod and one additional seta on outer margin of 1^st segment of endopod, protopod with 2 setae on coxa and 5 setae on basis. Duration of this sub stage was 24 to 36 hours. 

Mysis I

MTL: 3.36 mm (3.07-3.65 mm); MCL: 1.17 mm (1.12-1.26 mm).

Larvae assume more or less a shrimp like appearance in this stage, rostrum long and curved extending beyond eye, devoid of rostral spines, supraorbital prominent, a small spine present at anteroventral angle of carapace, hepatic spine well developed, carapace covers thoracic region completely and thoracic appendages are well developed; posterolateral spines persist on 5th and 6th abdominal segments, dorsal spines present on posterior margin of 4th, 5th and 6th abdominal segments, in some specimens on 3rd segment also, in rare cases even the 1st and 2nd abdominal segments possess a dorsal spine; minute pieopod buds seen on 1st five abdominal segments; 6^th abdominal segment develops a ventromedian curved spine at junction withtelson,ventrolateral spines on posterior end retained; telson broader distaliy with a median notch, each lobe bearing 2 lateral and 6 terminal setae, cleft extends to level half way between origin of outermost and penultimate pair of setae.

A1 with 3 segmented peduncle, I st segment longest with a ventromedian serrated spine, base of this segment swollen due to developing statocyst and carries 2 short plumose setae, numerous setae occur along appendage, distal segment carries 2 unsegmented rudiments offlagellae, inner one small and knob like carrying 1 very long and another short seta at its apex, outer flagellum carries on distal margin 3 setae and 4 aesthaetes; A2 with endopod unsegmented carrying 3 terminal setae, one proximal seta on inner margin and 2 small setae near a very small knob-like projection on inner side distaliy, exopod unsegmented, leaf like, with a distolateral seta on outer margin and II setae on distal and inner lateral margin; Md asymmetrical, with 7 free standing teeth in left Md and 3 in right Md, molar part shows a number of hard ridges bearing small teeth; Mx1, proximal segment of protopod with 8 setae; Mx2 with exopod enlarged to form scaphognathite carrying 10 plumose setae, proximal one being long and thick; Mxp1 with some setae on inner side of protopod longer and stouter, setae on coxa reduced to 5, exopod with 12 plumose setae, one seta each added to outer margin of 1st and 2nd segments of endopod: Mxp2 with 7 setae on basis of protopod, exopod as long as endopod carrying only 6 setae, 4 apical and 2 subapical, endopod 4 segmented, first 2 segments carry 1 seta on the outer side, terminal segment with 5 setae; Mxp3 well developed, protopod with 3 setae on basis, coxa without seta, endopod 5 segmented, terminal segment with 1 short and 5 long setae, 1st, 2nd and 4th segments each with 2 setae, 3rd segment naked, exopod as long as endopod carrying 4 apical and 3-4 subapical plumose setae; P considerably enlarged and their exopods serve as main swimming organs; PI, P2 and P3 almost identical, endopod segmentation indistinct, developing chelae with 5 long slender setae, exopod twice length of endopod with 4 apical and 3-4 subapical plumose setae; P4 and P5 almost identical, endopod unsegmented, half size of exopod, and bears 4 long setae apically, exopod with 4 long apical and 2 subapical setae uropods well developed, protopod with a large posteroventral spine, exopod with a prominent posterolateral spine followed by a short nonplumose seta and about 15 plumose setae along distal and inner margin, endopod with 14 plumose setae along inner and distolateral margin. Duration of this sub stage was 48 to 72 hours. 

Mysis II

MTL: 3.50mm (3.39-3.58mm); MCL: 1.20mm (1.15-1.26mm).

Presence of a spine on scaphocerite and appearance of unsegmented pieopod buds distinguish this sub stage from mysisI; no change in spination of carapace and abdomen; cleft on telson extends to level of origin of penultimate pair of lateral telsonic setae.

A1 with increased number of setae on peduncle, inner flagellum has increased in length and outer flagellum which is longer than inner with 6 aesthaetes and 1 or 2 setae at distal end; A2 with a small ventral spine on outer distal end of 2nd segment of protopod, endopod nearly half length of exopod bearing a short apical seta, exopod with 19 long plumose setae along inner and distal margin and 1 spine at distal lateral angle; Md with small unsegmented palp, 8 free standing teeth on left and 3 on right Md; Mx1 without exopod, size of endopod reduced; Mx2 with 14 to 15 plumose setae on exopod; Mxp1 with 12 setae on exopod; Mxp2 with 5 segmented endopod, with newly added segment in middle without setae, terminal segment with 6 setae; Mxp3 with endopod longer than exopod, 3rd segment with 2 setae, a seta added to outer distal margin of 4th segment; PI P2 and P3 almost identical endopod 5 segmented, distal segment with 2 and penultimate segment with 3 long setae, endopod of P4 and P5 4 segmented, distal segment with 2 apical and 1 subapical setae, penultimate segment bears 2 setae; pleopods have a slight constriction in the middle indicating the beginning of segmentation; exopod and endopod of uropod with 18 setae. Duration of this sub stage was 24 to 48 hours. 

Mysis III

MTL: 3.90 mm (3.43-4.17 mm); MCL: 1.26 mm (1.12-1.37 mm).

Development of 2 segmented pleopod bud distinguishes this sub stage from mysis II, no change in spination of carapace and abdomen, but a very minute rudiment of rostral tooth may be seen in a few specimens; telson long and rectangular carrying 6 distal and 2 lateral setae on each side, cleft extending to level of origin of 3rd pair of setae.

A1 statocyst clearly seen, both flagella are of equal size, inner unsegmented, bearing 4 long slender setae apically, of which one is longer, outer flagellum 2 segmented with 6 to 7 aesthaetes and 2 setae on the distal segment and 2 aesthaetes on the proximal segment; A2 with 2 segmented endopod carrying a long seta apically, exopod with 21 to 22 plumose setae and one anterolateral spine; Md still asymmetrical, palp long, but unsegmented; Mx2with 19 setae on exopod, rudiments of gills present as protuberance on bases of protopod of Mxp; Mxp1 with 12 setae on exopod; Mxp2 with an outer distal seta added to 4th segment of endopod; Mxp3 with endopod longer than exopod, distal segment with 1 short and 3 long setae; PI with rudiment of gill developed at base of protopod, endopod 5 segmented, chela as long as the other 3 segments put together, dactylus apically bearing 2 long setae, propodus with 2 setae at its joint with dactylus; P4 and P5 are identical, exopod as long as endopod, distal segment of endopod with 2 slender setae apically; exopod with 4 apical and 3-4 subapical plumose setae; pleopods 2 segmented and non-setose; distally some pleopods have developing setae uropod with 22 setae on exopod and 21 setae on endopod. Duration of this sub stage was 24 to 48 hours. 

Post Larvae I

MTL: 5.03 mm (4.55 - 5.26 mm); MCL: 1.53 mm (1.44- 1.61 mm).

Rostrum with 1 or 2 dorsal spines, supraorbital, hepatic and pterygostomial spines present, the latter often very small, median dorsal spines usually present on 4th, 5th and 6th abdominal segments, lateral spines present on 5th and 6th abdoming segments, anal spine still present on i6th abdominal segment, exopods of P small and without setae, pleopods well developed and setose, telson rectangular in shape carrying 3 pairs of lateral and 5 pairs of terminal setae' median notch practically absent.

Al with statocyst at base of 1^st segment, well developed ventromedian spine still present on basal segment; inner branch of distal segment 3 segmented, longer than outer and carries 4 setae apically, of which 1 is as long as the branch, outer branch 2 segmented carrying 8 aesthaetes and 3 setae; A2 with endopod 6 segmented, distal segment apically bearing 3 long and 3 short setae; exopod with 27 setae and one anterolateral spine; Md has become almost symmetrical, free standing teeth lost, palp well developed and 2 segmented, carrying setae; Mx1 with endopod much reduced, unsegmented and without setae; distal lobe of protopod larger than proximal, distal and proximal lobes with 13 to 18 and 7 to 8 setae respectively; Mx2 with much reduced protopod having 4 endites, proximal 2 endites with 2 setae, distal 2 endites carry 5 to 6 bristle like setae, endopod reduced, unsegmented, without setae, scaphognathite very conspicuous bearing 29 to 30 plumose setae; Mxp1 with endopod and exopod reduced in size without segments and setae, protopod has become wide with numerous setae, epipod well developed; Mxp2 with exopod vestigial, endopod recurved, distal segment with 6 spine-like setae, penultimate segment naked, protopod carries a gill; Mxp3 with gill on protopod, exopod rudimentary without setae, 4th segment of endopod with 5 setae; PI with rudimentary gill on protopod, exopod much reduced, without setae, chelae fully developed; P2 and P3 progressively longer than PI; P4 and P5 almost similar, exopod reduced, endopod 5 segmented, 3rd and 4^th segments carry 3 setae each; 26 to 27 setae on margin of exopod and endopod of uropod. Duration of this sub stage was 24 to 30 hours.

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