Prawn: External Features and Life-History

In this article we will discuss about:- 1. Habitat and External Feature of Prawn 2. Appendages of Prawn 3. Movement 4. Alimentary System 5. Respiratory System 6. Vascular System 7. Excretory System 8. Nervous System 9. Receptor Organs 10. Reproductive System 11. Breeding and Life-History.

Contents:

1. Habitat and External Feature of Prawn
2. Appendages of Prawn
3. Movement of Prawn
4. Alimentary System of Prawn
5. Respiratory System of Prawn
6. Vascular System of Prawn
7. Excretory System of Prawn
8. Nervous System of Prawn
9. Receptor Organs in Prawn
10. Reproductive System in Prawn
11. Breeding and Life-History of Prawn

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1. Habitat and External Feature of Prawn:

The fresh-water prawn Palaemon is commonly found in rivers, streams and ponds all over India, Bangladesh and Pakistan. It differs somewhat from the marine prawn Penaeus which belongs to a separate genus. Both are not commonly available in Calcutta market and relished as food. Palaemon is habitually a shy animal, hiding by-the day and coming to the surface of water at night.

A full-grown prawn usually measures about 6 to 8 inches (15 to 20 cms) in length. When freshly caught it is pale blue in colour. The body is elongated and cylindrical, but slightly compressed from side to side.

It is composed of two parts:

(1) An anterior, rigid, un-jointed cephalothorax and

(2) A posterior, movable, jointed abdomen which can be flexed forwards under the cephalothorax.

The abdomen is divided into six distinct segments, each bear­ing a pair of appendages on its ventral surface. At the posterior end of the abdomen, there is a terminal conical piece called telson which is devoid of appendages. The cephalothorax is formed by the fusion of the head and thorax. It bears thirteen pairs of appendages on its ventral surface.

Taking into accounts that each abdominal segments typically bears pair of appendages it can easily be inferred that the cephalothorax is composed of at least thirteen appendage-bearing segments. It has been proved conclusively that the first five cephalothoracic segments belong to the head and the remaining eight to the thorax.

Exoskeleton:

The soft parts of the prawn are protected by an armour of thick cuticle which is hardened by the deposition of lime salts (CaCO₃). The resulting structures constitute the exo­skeleton of the prawn. The cephalothorax is covered dorsally by a huge-shield-shaped carapace and ventrally by a number of hard calcified sheets called sternal plates.

The carapace or dorsal shield is produced forward as a long median outgrowth called rostrum which bears teeth- like serrations on its upper and lower margins. At the base of the rostrum, on each side of the carapace, there is a distinct orbital notch for accommodating the eye.

The paired eyes are mounted on movable eye-stalks and are lodged in the notch when turned outwards. There are two small spines below and behind each eye- stalk; of these two, the anterior spine is longer and is known as the antennal spine and the shorter posterior one is the hepatic spine.

The carapace hangs down freely on either side of the cephalothorax, thus forming a convex outer covering for the narrow-gill-chamber. Each of the gill-covers is known as a branchiostegite. Each abdominal segment is covered by a ring-like calcified piece called sclerite. The six successive sclerites are connected with one another by thin unclassified sheets called arthrodial membranes which permit movement of the sclerites.

A sclerite is com­posed of the following parts. The thick arch-­like dorsal part of the sclerite is the tergum, and the thin plate-like ventral part, placed transversely beneath the abdomen, is the sternum. Each side flap of the tergum hangs freely down and is known as the pleuron. The pleuron is connected to the appen­dage of the same side by a small chitinous plate called epimeron.

The tergum and pleura of each sclerite slightly overlap those of the succeeding sclerite; thus producing an im­bricate arrangement. The sclerites are articu­lated with one another by five hinge-joints which are distinctly vis­ible on each side of the body as orange-colour­ed spots. The hinge- joints permit flexion and extension of the abdo­men in a vertical plane.

Typically there is one pair of appendages in each segment of the body. The prawn carries, in all, nine­teen pairs of appen­dages of which the first five are cephalic, the following eight are tho­racic, and the remaining six are abdominal. Each of the first two segments bears a pair of prehensile antennae or feelers. The first antenna or antennule lies behind the eye. The second an­tenna or antenna is just posterior to the antennule.

The antennuies and antennae are pre-oral in position, that is, they lie in front the mouth. Following the antennae, there is a toothed appendage called mandible on each side of the mouth; these are used for cutting and crushing food.

Behind the mandibles, there are two pairs of leaf-like jaws, the first maxilla or maxillula followed by the second maxilla or maxilla. Next to the jaws, there are three pairs of maxillipedes and five pairs of walking legs. The first and second maxillipedes are flat and foliaceous, while the third maxillipedes and the walking legs are elongated and pediform.

The jaws and the maxillipedes are employed for pushing food into the mouth, whereas the walking legs are used for crawling on the river bed. Of the five pairs of walking legs the first two pairs bear pincers at their free ends and are, therefore, called chelate legs; these along with the rostrum and the spines are regarded as organs of offence and defence.

The six pairs of abdominal appendages are known as pleopods or swimmerets; they help the prawn in swimming.

Apertures:

There are two median and three paired apertures, opening on the surface of the body. The mouth is a slit-like opening on the ventral surface of the cephalothorax between the third and the fourth segment; it serves for the ingestion of food. The anus is a longitudinal slit placed upon the ventral surface of the telson near its base; it serves for the egestion of faeces.

The gonopores or reproductive openings differ in position in the two sexes of the prawn. The male gono­pores are paired apertures, one at the base of a fifth walking leg. The female gonopores, however, are found at the bases of the third walking legs. As the prawn is unisexual, the male and female gonopores are never found in the same animal.

The paired renal apertures or urinary openings are situated internally at the bases of the antennae (second antennae). Besides these, there are two minute openings, one on each antennule (first antenna), by which the statocysts communicate with the exterior. The statocysts are the organs by which a prawn maintains its equilibrium.

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2. Appendages of Prawn:

The appendages of the prawn differ widely for performing different functions. The modification takes place either by varia­tion of shape or by suppression of one or more parts. But all the appendages are built on the same general plan. An appendage is said to be biramous because it bears two branches or rami.

A typical biramous appendage consists of:

(1) A two jointed stalk of protopodite, the two segments of which are the coxa and the basis; the coxa lies proximal to the basis;

(2) Attached to the basis, there are two branches or rami, an outer exopodite and an inner endopodite. The proximal end of the coxa is lodged into a depression on the ventral surface of the animal, thus form­ing a ball and socket joint (Fig. 89 A).

There are, as already indicated, nineteen pairs of appendages. According to their position on the body, the appendages are divi­ded into cephalic, thoracic and abdominal. Of the thirteen pairs in the cephalothoracic region, the first five are cephalic and the follow­ing eight are thoracic. The remaining six pairs on the abdomen are abdominal. Appendages of one side are described below.

Cephalic Appendages:

There are five pairs of cephalic appendages: the antennules, the antennae, the mandibles, the maxillulae and the maxillae. The antennule (first antenna) is situated in front of the mouth just behind the base of the eye-stalk. Its protopodite consists of three segments: precoxa, coxa and basis.

The procoxa is very large and carries a spine on its outer side. There is a shallow depression on its dorsal surface, bearing the opening of the statocyst. The statocyst is a small cuticular sac which lies within the precoxa; it is the balanc­ing organ of the prawn. The coxa is short; it is provided with sen­sory hairs or setae.

The basis is slightly longer than the coxa and carries a pair of many jointed feelers or flagella. The outer feeler is divided into two branches, of which the smaller inner branch contains olfactory setae for the purpose of smelling. The two feelers are modified exo and endopodites of the protopodite.

The antenna (second antenna) is also pre-oral in position. Its peduncle or protopodite consists of two segments: coxa and basis. The excretory organ or green gland is lodged inside the coxa, and the urinary opening or renal aperture is situated on its inner margin.

The basis gives off two rami—a leaf-like scale or squama and a long many-jointed flagellum or feeler. The squama represents the exopo­dite and help the prawn as a balancer in swimming. The flagel­lum bears tactile setae and represents the endopodite.

Mandible:

It is the third appendage and lies on the outer side of the mouth. Its spoon-shaped proximal part is called the apophysis, while the solid distal part is the head. The head consists of a stout molar process and a thin incisor process.

The molar process is placed at right angles to the apophysis and carries 5 to 6 yellowish- brown teeth. The incisor process is placed in front of the oral aperture and bears three closely set whitish teeth. A three-jointed mandibular palp arises from the outer side of the head; it bears sensory setae and lies against the outer side of the incisor process.

The coxa of the protopodite is modified to form the apophysis and the head of the mandible. The basis of the protopodite corresponds to the proximal segment of the palp and the endopodite is repre­sented by the two distal segments of the same. The exopodite is absent. The mandible serves as a cutting and crushing apparatus.

Maxillula (First Maxilla):

It is the smallest of all the ap­pendages and is placed behind the posterior lip of the mouth. The maxillula consists of three foliaceous plates, the margins of which are covered with sensory setae.

Of the three plates, the two directed inwards are the jaws or gnathobases; these are the coxa and basis if the protopodite. The third, directed outwards represents the endopodite. The exopodite is absent. The maxillula helps to push bed into the mouth.

Maxilla (Second Maxilla):

It is a thin foliaceous appendage placed behind the maxillula. Its peduncle consists of a much educed coxa and a larger bifurcated basis which projects inward is the endites or jaws. The large fan-shaped exopodite is known is the scaphognathite; it lies in front of the gill-chamber.

The small endopodite is placed between the exopodite and the endites. The maxilla has a two-fold function. The endites are used to push bod into the oral cavity. The scaphognathite helps in respiration by maintaining a constant flow of water through the gill-chamber.

Thoracic Appendages:

These consist of three pairs of maxillipedes or foot-jaws and five pairs of walking legs.

The first maxillipede has a protopodite consisting of two segments, coxa and basis. These are flattened leaf-like structures carrying stiff setae on their inner margins. They push food into the mouth. Attached to the outer side of the coxa, there is a bilobed structure called epipodite which helps in respiration. The endopodite s short but the exopodite is long and provided with a thin expanion at its base.

The second maxillipede is less foliaceous than the first. Its protopodite consists, as usual, of coxa and basis. The coxa is short and covered with setae on its inner margin; it carries a small epiodite and a gill on its outer margin.

The basis is immovably articulated to the endopodite. The exopodite is long, slender, un-jointed and whip-like. The endopodite consists of five segments, lamely, ischium, merus, carpus, pro-podus and dactylus. The proodus and the dactylus are bent and turned backwards forming a knife-like cutting plate.

The third maxillipede is pediform or leg-like in appearance. Its protopodite consists of coxa and basis. The coxa carries a thin pipodite on its outer side. There is a slender un-jointed exopodite attached to the outer side of the basis and bearing stiff setae.

The endopodite is three-jointed—ischium being fused with the merus, and pro-podus with dactylus, while the carpus remains free, forming the middle segments of the end and pro-podus with the dactylus, while the dactylus, while the remain free, forming the middle segment of the endopodite.

Walking Legs:

The prawan is decapod (deca= ten) bearing ten walking legs. Even walking leg consists of seven cylindrical segments joined end to end. The first two segments represent the coxa and basis of the protopodite and the remainder represent the five segments of the endopodite. The expodite is absent. The five segments of the endopodite are: ischium, merus, carpus, pro-podus and dactylus, the last being clawed.

The first and second walking legs carry pincers at their proximal ends; hence they are known as the chelate legs. The last three legs are non-chelate. The second walking leg is very much larger and bears spines and setae.

It is, therefore, known as the large chela, whereas the first walking leg is known as the small chela. The walking legs help the prawn to crawl upon the river bed. The chelate legs are organs for offence and defence and help in holding the food before being taken. Gonopores are situated at the bases of the third walking legs in female; but in male, they lie at the bases of the fifth walking legs.

Abdominal Appendages:

There are six pairs of abdominal appendages called pleopods or swimmerets which help the prawn in swimming. The last pair are specially called uropod’s because they form with the telson a powerful tail-fin which is used for leaping backward.

Each pleopod is a typical biramous type of appendage. Its protopodite consists of a short proximal coxa and an elongated distal basis. The basis carries a pair of leaf-like rami, the outer exopodite and the inner endopodite. Both the rami are covered with tactile setae. The endopodite is smaller than the exopodite.

Each of the second, third, fourth and fifth pleopod bears on the inner side of its endopodite a hook-like rod called appendix interna. The appendices internae of the two sides interlock with one ano­ther during the breeding season, thereby forming a efficient basket for carrying eggs. The second pleopod, in male, bears another additional rod-like process, called appendix masculina (Fig. 89 B).

The uropod has a protopodite of only one segment. The rami are oval and fan-like; exopodite is divided by a fine suture and bears a spine near its base, but the endopodite is undivided. Both the rami are covered with setae along their margins.

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3. Movement of Prawn:

Prawn is capable of three kinds of locomotion: walking, swim­ming and darting. In walking, the body is held straight, the abdomen is extended, the five pairs of walking legs remain in contact with the substratum and moved in an orderly manner. The pincers are held in front and the feelers of the antennules and the antennae are in continuous motion making a survey of the environment.

In swimming, the animal progresses through the water by rapid paddle-like movement of its swimmerets. In darting, the prawn flexes. Its abdomen under the cephalothorax.

It then violently extends the abdomen putting pressure on the water by its fully expanded oar-like tail-fin. This is effected so quickly that the entire animal jerks backward, covering a con­siderable distance. The movement is repeated frequency whenever the animal apprehends danger. It is, therefore, known as the escape movement.

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4. Alimentary System of Prawn:

The alimentary system is composed of the alimentary canal and the digestive gland.

The alimentary canal extends from mouth to anus, and con­sists of three distinct parts:

(1) Foregut or stomodoeum,

(2) Mid­gut or mesenteron, and

(3) Hindgut or proctodoeum.

The foregut and the hindgut are lined internally by a thick cuticle which is continuous with the outer covering of the body; the midgut, how­ever, is lined by a soft layer of endoderm. The foregut includes mouth, buccal cavity, oesophagus, and stomach; the midgut is the long and narrow intestine; the hindgut consists of the terminal bulb or rectum and the anus.

The mouth is a large opening on the ventral side of the cepha­lothorax between the third and the fourth segment. It is bounded anteriorly by a shield-shaped labrum, posteriorly by the labium, and on each side by the incisor process of a mandible.

The buccal cavity is a short and narrow chamber which connects the mouth with the oesophagus. The oesophagus is a wide rectangular tube running vertically upwards to join the stomach.

The stomach occupies the greater part of the cephalothorax; it consists of two parts:

(1) A large bag-like cardiac stomach, and

(2) A small narrow pyloric stomach placed behind and beneath the former. Two parts of the stomach are separated from one another by a number of valves. The inner wall of the cardiac stomach is raised into ridges and strengthened by hard cuticular plates covered with bristles; these help in grinding and guiding food towards proper channel. Thus, there is a circular plate, covering the oesophageal opening in the anterior part of the cardiac stomach and imme­diately behind this, but on the roof, there is a lanceolate plate.

On the floor of the cardiac stomach and lying in the mid-ventral line, there is a triangular hastate plate resembling the head of a spear. The broad base of the hastate plate extends posteriorly, forming the anterior margin of the cardio-pyloric opening.

A curved com­bed plate is found on each side of the hastate plate; it carries deli­cate bristles on its inner margin, thereby resembling a comb. A narrow tunnel called lateral groove lies on either side of the hastate plate; it is a drain pipe covered by the thickly set bristles of the combed plate and leading into the cardio-pyloric opening.

Thus a filter bed is formed on each side of the hastate plate, permitting the passage of finely divided food particles into the underlying drains. On the outer side of each combed plate, there is a raised longitudinal fold called guiding ridge.

They are so named because they serve to direct the contents of the cardiac stomach towards the cardio-pyloric opening. The pyloric stomach is very small in comparison to the huge cardiac stomach.

It receives food either in solution or in a fine state of division through the cardio-pyloric opening. Digestive juice is poured into the pyloric stomach by two hepatopancreatic ducts, one on each side. The pyloric stomach leads into the midgut or intestine which is the longest part of the alimentary canal. It is usually found as a narrow black tube running towards the posterior end as far back as the sixth abdo­minal segment.

It lies in the mid-dorsal line, just beneath a thin layer of extensor muscles. The supra-intestinal artery accom­panies the intestine throughout its course and lies dorsal to it. The midgut is continued into the short hindgut, the first part of which is dilated to form an intestinal bulb or rectum. The rectum narrows down to the anus—a slit-like opening on the ventral surface, near the base of the telson.

Hepatopancreas is the only digestive gland of the prawn. It is a large orange-coloured body with soft and fragile tissues, occupying a considerable portion of the cephalothorax. The pyloric part of the stomach and the commencement of the midgut are completely surrounded by the hepatopancreas. The gland has two lobes, from each of which arises a hepatopancreatic duct.

These open separately into the pyloric stomach and carry digestive juice secreted by the gland. The hepatopancreatic juice contains enzymes for digesting proteins, fats and carbohydrates. Digested food is partly absorbed by the gland through its ducts and stored for future use. The hepa­topancreas, therefore, not only secretes digestive enzymes but also absorbs digested food and serves as an important storage-organ.

Mechanism of Feeding and Digestion:

Prawn feeds upon mosses, algae, insects and decaying organic matter. It feeds actively at dusk and just before daybreak. Food is seized by the chelate legs and pushed into the buccal cavity by the maxillipedes, maxillulae and maxillae.

The food is cut into small bits and crushed by the mandibles. A series of wavy peristalsis push the food along the oesophagus towards the cardiac stomach, the walls of which con­tract and expand alternately. The food is thereby sucked into the stomach.

The food is thoroughly churned and to a certain extend crush­ed by the contraction and expansion of the sac-like cardiac stomach against the cuticular plates in its interior. Small particles of food are strained through the bristles of the combed plate into the lateral grooves, which act as drain pipes and guide the same into the pyloric stomach.

The food is digested by the action of the digestive juice from the hepatopancreas. Absorption of the digested food takes place in the intestine and residue is egested through the anus. A part of the digested food is directly absorbed by the hepa­topancreas and stored there for future use.

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5. Respiratory System of Prawn:

Prawn is adapted for utilizing oxygen dissolved in water for the purpose of breathing. The blood of this animal contains a res­piratory pigment called haemocyanin, which acts in the same way as haemoglobin of other animals.

The respiratory organs of the prawn consist of:

(1) The lining membrane of the branchiostegite,

(2) Three pairs of epipodites, and

(3) Eight pairs of gills.

These are situated in the gill-chambers, one on each side of the cephalothorax. The gill-chamber is bounded externally by the branchiostegite and internally by the cephalothoracic wall; it is closed dorsally but communicates with the exterior along its anterior, posterior and ventral borders. Respira­tory organs of one side are now described.

(1) The highly vascular and thin lining membrane of the branchiostegite forms a huge respiratory surface through which O₂ is readily taken in and CO₂ is given out.

(2) The epipodites are small leaf-like membranes attached to the bases of the maxillipedes. They are thin and vascular and therefore take part actively in carrying out respiration.

(3) The gills are situated in relation to the thoracic appendages.

Out of the eight gills, seven are directly exposed by cutting the branchiostegite away, but the third remains hidden under cover of the second gill. The gills are more or less crescent-shaped in out­line. They increase in size anteroposteriorly, so that the first gill is the smallest and the last one is the largest. Each gill has a long narrow base supporting two rows of flat and rectangular gill-plate which are arranged like the leaves of a book.

With reference to its position and point of attachment, a gill may be either podobranch or arthrobranch, or pleuro-branch. The first gill is attached to the coxa of the second maxillipede; hence it is a podobranch.

The next two are attached to the arthrodial membrane articulating the third maxillipede with the body; hence, these two are arthrobranchs. The remaining five gills are pleurobranchs, because they are attached to the outer side of the cephalo thorax above the arti­culation of the walking legs.

Each gill receives deoxygenated blood through an afferent branchial channel, and aerated blood leaves the gill by an efferent branchial channel. The afferent and efferent channels are joined by narrow connectives which are distributed on the gill-plates.

Mechanism of Respiration:

Constant supply of oxygenated water to each gill-chamber is maintained by the paddle-like move­ment of the scaphognathite of the maxilla. Water rushes in along the posterior and ventral borders of the branchiostegite, and is expelled from the anterior end of the gill-chamber by the baling action of the scaphognathite and by the vibratory motion of the exopodites of the maxillipedes.

The gills, the epipodites and the lining membrane of the branchiostegite are richly supplied with blood. All of them are constantly bathed in fresh-water containing dissolved oxygen. The gaseous exchange between the blood and the water takes place by diffusion—oxygen being taken in and carbon dioxide given out.

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6. Vascular System of Prawn:

As in all arthropoda, the vascular .system is open and the blood vessels freely communicate with spaces surrounding the viscera. These spaces together form the body cavity which is full of blood and is, therefore, not a true coelom but a hoemocoele. The true coelom, however, is restricted to the cavities of the gonads.

The vascular system of prawn consists of:

(i) The heart sur­rounded by the pericardium,

(ii) The arteries or true blood vessels with definite muscular wall, and

(iii) The irregular sinuses or blond lacunae into which the arteries open directly.

Heart:

The heart is roughly triangular in shape. It is placed in the mid-dorsal line at the posterior part of the cephalothorax. The pointed apex of the heart is directed forwards and the broad base is directed backwards. It lies beneath the carapace, dorsal to the gonads, and is held in position by a muscular cord, the cardio- pyloric strand, which binds it down to the pyloric stomach.

The heart is enclosed by a special hoemocoele, the pericardial sinus, the thin wall of which forms the pericardium. The muscular wall of the heart is pierced by five pairs of slit-like openings called ostia (ostium) =door). Each ostium is a one-way passage permitting blood to flow from the pericardial sinus into the heart. The lips of the ostia act as valves and prevent leakage in a reverse direction.

Of the five pairs of ostia, one pair is found on the dorsal surface, another pair on the ventral surface, a third pair on the posterior border, and the remaining two pairs pierce the lateral borders of the heart. The cavity of the heart is traversed by a large number of interlacing muscle fibres; it is not a continuous cavity and appeal’s to be spongy in structure.

Arteries:

Receiving blood from the pericardial sinus, the heart pumps out the same into the arteries which are vessels with strong muscular wall.

One median and two paired arteries originate from the apex of the heart.

These include:

(a) A median ophthalmic artery,

(2) Paired antennary arteries,

(3) Paired hepatopancreatic arteries.

The slender median ophthalmic artery runs forward up to the roof of the oesophagus, where it ends by anastomosing with the antennary arteries; it lies beneath the carapace and along the mid- dorsal line. The paired antennary arteries arise from the apex of the’ heart, One on either side of the median ophthalmic.

While passing forward, each antennary artery supplies:

(1) A pericardial branch to the pericardial sinus,

(2) A gastric branch to the stomach, and

(3) A mandibular branch to the mandibular muscle.

Giving off these three branches, the antennary artery extends up to the base of ‘the antenna of its own side and breaks up into a dorsal and a ventral division. The dorsal division turns inward, supplies an optic artery to the eye, and finally ends by anastomosing with its fellow of the opposite side and with the median ophthalmic.

An arterial circle is thus formed anteriorly which gives off a pair of rostral arteries to the rostrum. The ventral division of the antennary artery breaks up into two branches, one supplying the anten­nule (first antenna) and the other supplying the antenna (second antenna).

The paired hepatopancreatic arteries originate from the ventral surface of the apex of the heart. Each lies on the outer side of the corresponding antennary artery and plunges directly downwards to supply the hepatopancreas.

A single median artery arises from the postero-ventral surface of the heart.

It soon divides into two branches:

(1) A slender supra- intestinal artery, and

(2) The stout sternal artery.

The supra-intes­tinal artery runs towards the posterior end, lying dorsal to the midgut; it extends as far back as the rectum and supplies branches to the intestine and dorsal muscles. The sternal artery is the stoutest of all the blood vessels; it plunges obliquely downwards and passing through an aperture in the thoracic ganglionic mass of the ventral nerve-cord it comes to lie on the ventral side.

The artery now divides into two branches:

(1) A large ventral thoracic artery which extends forwards up to the mouth, supplying blood to the ventral part of the cephalodiorax, and

(2) A ventral abdominal artery which runs backward up to the anus, supplying blood to hindgut and the ventral region of the abdomen.

The arteries ramify into all the parts of the body, forming small hair-like vessels which open directly into the hoemocoelic body cavity. These hair-like vessels with open mouth are not true capillaries, because true capillaries are always closed vessels occur­ring in the form of a network.

Blood-Sinuses and Blood Channels:

Blood flows through irregular spaces or lacunae and is finally received into a pair of ventral sinuses. Each ventral sinus is an irregular space beneath the hepatopancreas and extends lengthwise for a short distance into the abdomen. The two sinuses communicate with one another by several connecting channels.

From each ventral sinus blood is supplied to the gills of the same side by six afferent branchial channels. Of the six, the last five carry blood to the pleurobranchs, whereas the first supply blood to the two arthrobranchs and the single podobranch.

Aerated blood is brought back from the gills by six pairs of efferent branchial channels which drain into the dorsal or pericardial sinus. From the pericardial sinus the blood flows through the ostia into the heart.

Blood:

The blood is a clear fluid containing amoeboid leuco­cytes. Coloured erythrocytes are total absent. The respiratory pigment is a compound of copper with a protein substance, and is known as haemocyanin. It remains in a dissolved state in the blood plasma. When oxygenated, the blood appears to be pale blue; deoxygenated blood is colourless.

Course of Circulation:

Deoxygenated blood from blood-lacunae indifferent parts of the collected into two ventral sinuses. It flows to the gills through six pairs of afferent branchial channels. Blood is aerated in the gills and brought back into the pericardial sinus by six pairs of efferent branchial channels. From there, the blood enters the heart through five pairs of ostia. The heart, by systolic contraction, pumps the aerated blood into the arteries.

Leakage of blood into the pericardial sinus is prevented by the tightly closed lips of the ostia. Arteries ramify into distant parts of supplying the tissues with O₂. There is no capillary network; the hair-like branches of the arteries open directly into the blood lacunae. By this time, the blood has become deoxygenated. “It slowly collects in the ventral sinuses and then sent back to the gills for aeration.

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7. Excretory System of Prawn:

The excretory organs of the prawn consist of a pair of green glands, one inside the coxa of each antenna (second antenna). Because of their position, the green glands are also known as the antennary glands.

A green gland consists of three parts:

(1) An end-sac,

(2) A labyrinth, and

(3) A bladder.

The three parts are tightly com­pressed to form a small opaque body about the size of a pea-seed. The bean-shaped end sac, containing a blood lacuna, is placed in the middle, between the labyrinth and the bladder.

The labyrinth is the gland proper. It is situated on the outer side of the end-sac and is composed of a mass of excretory tubules opening into the bladder. The bladder is a thin-walled sac lying on the inner side; it communicates with the urinary opening by a short ureter. The urinary opening or renal aperture is situated on a raised papilla at the base of the antenna (second antenna).

The green glands are joined to a large median renal sac, which lies on the dorsal surface of the cardiac stomach just beneath the carapace. The renal sac is thin-walled and transparent. It is connected to the bladder of each green gland by a narrow tube called lateral duct.

Ammoniacal compounds along with excess of water arc separated from the blood in the end-sac, and collected in the form of urine by the excretory tubules of the labyrinth. The tubules carry the urine into the bladder.

The urine is voided through the renal apertures, the renal sac serving as a temporary reservoir. The integument of prawn actively separates nitrogenous waste products which are stored in the hard outer shell of its body and eliminated at the time moulting. The skin, therefore, is an ac­cessory excretory organ.

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8. Nervous System of Prawn:

The nervous system of the prawn is built on the same plan as that of the leech.

It is com­posed of three parts:

(1) Central nervous system,

(2) Peripheral nerves, and

(3) Sympathe­tic nervous system.

The central nervous system consists of:

(i) A pair of supra-oesopha­geal ganglia form­ing the brain and connected by

(ii) A pair of circumoesophageal commissures with

(iii) A ventral thoracic ganglionic mass, and

(iv) A median ganglionated ventral nerve-cord, extending backwards from the thoracic ganglionic mass up to the base of the telson (Fig. 96).

The brain is a bilobed mass slightly cut in the middle to form the two supra-oesophageal ganglia. It lies dorsally beneath the base of the rostrum, at the junction of the oesophagus with the cardiac stomach. Each supra-oesophageal ganglion supplies nerves to innervate the eye, the antennule, and the antenna of its own side.

(1) The optic nerve enters the eye-stalk from the outer side of the ganglion and supplies the retinal region of the eye.

(2) The antennular nerve arises anteriorly; it enters the base of the antennule and breaks up into branches to supply the different parts of the append­age, including the statocyst which is lodged inside the pre-coxa.

(3) The stout antennary nerve originates pos­teriorly; it runs ob­liquely backward and the curves forward to enter the base of the antenna. It sends a branch to the green gland and then breaks up to inner­vate the different parts of the append­age.

The circumoesophageal com­missures are stout cords which connect the brain with the thoracic ganglionic mass. Arising from the posterior parts of the brain, the two cords run backwards and slightly down­wards to embrace the oesophagus.

They meet posteriorly in the mid-ventral line to join the thoracic ganglionic mass. There is a slender nerve loop connect­ing the two commis­sures just before they meet behind the transverse commissure is known as the post-oesophageal loop. A small nerve originates from the posterior part of each commissure to supply the mandible of its own side.

The thoracic ganglionic mass is a large oval body lying at the commencement of the ventral nerve-cord, in the mid-ventral line of the cephalothorax. It is composed of a number of ganglia which are fused together and cannot be distinguished as separate structures.

As many as eleven pairs of nerves arise from the ganglionic mass and supply the cephalothoracic appendages ex­cepting the antennules and the antennae. The sternal artery passes through a small opening in the posterior part of the ganglionic mass.

The ventral nerve-cord lies beneath the mass of abdominal muscles. Beginning from the posterior end of the thoracic gang­lionic mass, it extends along the mid-ventral line and terminates posteriorly near the base of the telson.

Corresponding to the number of abdominal segments, there are six ganglionic enlargements in the course of the abdominal part of the nerve-cord. Each such ganglion is really composed of two fused ganglia. In fact the ventral nerve-cord, although it appears to be outwardly single, is the product of fusion of two cords placed side by side.

Each abdominal ganglion supplies branches to innervate the muscles and the appendages of its own segment. The sixth abdominal ganglion is the largest of the lot and supplies, in addition, the hindgut and the muscles of the telson.

The peripheral nerves originate from the ganglia of the central nervous system. They have been already described. Each nerve is composed of a bundle of nerve fibres. The nerve fibres are of two kinds: afferent or sensory and efferent or motor.

The afferent fibres are distributed to the receptor organs such as eyes, etc. The efferent fibres, on the other hand, innervate the voluntary muscles and the glands. Most of the nerves are mixed containing both kinds of fibres.

The sympathetic nervous system consists of a few small ganglia connected by slender nerves. These are situated chiefly on the dorsal surface of the cardiac stomach and supply branches to innervate the involuntary muscles of the viscera.

A fine sympathetic nerve arises from the posterior part of the brain and runs backwards along the roof of the cardiac stomach. It bears two small ganglia called visceral ganglia, one behind the ether. The anterior visceral ganglion is connected by a slender visceral loop with a pair of small commissural ganglia, lying along the course of the circumoesophageal commissures.

The posterior ganglion is free and gives out paired nerves. It is to be noted that the brain and the ventral nerve-cord of the prawn are solid structures without cavities. They, therefore, differ entirely from the hollow brain and the dorsal, tubular spinal cord of the toad.

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9. Receptor Organs in Prawn:

The sense organs consist of:

(1) Paired eyes,

(2) Paired stato­cysts,

(3) Tactile organs, and

(4) Olfactory setae.

The eyes are hemispherical in shape and situated at the anterior end of the cephalothorax, one on either side of the cara­pace, near the base of the rostrum. Each eye is mounted at the end of a two-jointed eye-stalk which can be moved forwards and back­wards, as well as upwards and downwards.

The eye is compound, because it is composed of a large number of a simple visual elements called ommatidia or ocelli. Such compound eyes are almost uni­versally found in all arthropod animals.

The outer surface of the eye is convex and roughly divided into a large number of square facets placed in juxtaposition like the squares on a graph paper. Each facet corresponds to a single ommatidium or ocellus (Fig. 97A). The ommatidia are arranged regularly along the radii of the hemispherical eye. (Fig. 97B).

Each ommatidium is a complete apparatus for the reception of light rays and is composed of a number of cells placed end to end along a central axis. Immediately beneath the cuticular corneal facet, there is a pair of flat corneagen cells which can secrete a new cornea as soon as the old one is lost or damaged in any way.

Beneath the corneagen cells lies a transparent body called crystalline cone or lens. The crystalline cone is surrounded by four tall cells, the cone cells or vitrellae, which secrete and nourish the lens.

The proximal part of the ommatidium is composed of a spindle-shaped, transversely striated body called rhabdome, which lies beneath the lens and is completely surrounded by seven long cells, the retinular cells. The latter secrete and nourish the rhabdome.

The optic nerve, on entering the eye-stalk, breaks up into branches which supply the retinular cells and the rhab­dome. Each ommatidium is optically isolated from the neighbouring ommatidia by dark pigment sheaths composed of chromatophores.

These are arranged in two groups:

(i) The proximal group surrounding the rhab­dome and retinular cells is known as the retinal sheath;

(ii) The distal group surrounding the lens and vitrellae consti­tutes the iris-sheath. The pigments sheaths are capable of extension and retraction according to the intensity of the avail­able light. In dim light and during the night the pigment sheaths are retracted, but in bright light and during the day the sheaths are fully extended.

The rhabdome and the retinular cells, together with the retinal sheath constitute the receptor or retinal part of the eye. The cornea, corneagen cells, crystalline cone, vitrellae and the iris sheath are concerned with focusing of light on the retinal part. They, therefore, form the dioptrical region of the eye. The light received by the corneal facet is focused on the retinal part and the latter transmits the same to the brain.

It is to be noted that each ommatidium is capable of producing an image; when the ommatidia are optically isolated from one another by extension of the pigment sheaths, a composite image is formed, being made up of a large number of separate bits, each of which is the contribution of a single ommatidium.

This is known as the mosaic vision and the apposition image thus pro­duced is formed during the daytime in the presence of bright light. In dim light and during the night, the pigment sheaths are retracted and all the ommatidium work together to form a single complete image. The image in this case is indistinct and blurred. It is called a superposition image.

The statocysts or otocysts are paired organs for maintaining balance. Each statocyst is a nearly spherical cuticular sac about 1·5 mm. in diameter, and is lodged within the precoxa of an antennule. It is attached to the lower surface of the concave roof of the precoxa, and communicates with the exterior by a small opening.

Its essen­tial part consists of an oval ring of delicate cuticular setae which are situated in the centre of the sac. Each seta consists of a swollen base and a filamentous shaft, which is bent at the middle of its length. The free ends of the setae point towards the centre of the sac and are covered with fine bristles. Each seta receives at its base a fine branch from the statocystic nerve. (Fig. 100A).

There are numerous sand grains within the cavity of the statocyst which are introduced by the animal at the time of moulting. With alteration of position, the sand grains in contact with some of the sensitive setae and the stimulus of contact produces an impulse which is transmitted to the brain.

The animal, coming to know of the changed circumstances, quickly re-orients itself and regains its normal balance. That these organs are concerned with equilibration is evident from the fact that if they are removed the animal appears to lose all sense of balance and fails to perceive the direction of the force of gravity.

If freshly molted prawns are kept in an aquarium in which sand grains are replaced by iron filings, then lacking anything else, the animals put fine iron dust within their statocysts.

Such animals when subjected to the influence of a powerful electro-magnet, always swim with their ventral surface turned towards the instru­ment. This conclusively proves that the statocysts are the organs for perceiving the direction of the force of gravity.

The tactile organs are concerned with the perception of touch and contact. The many-jointed elongated feelers of the antennules and antennae are solely used for this purpose. Besides these, the borders of the flattened appendages, such as the rami of the pleopods, are covered with tactile setae.

Each tactile seta is a cuticular rod the shaft of which consists of two segments:

(1) A swollen base, and

(2) A plumose distal part tapering at its free end and bearing two rows of barbs (Fig. 100 C).

The olfactory setae are con­cerned with the perception of smell. They are chiefly found within the hollow of the small inner branch of the outer feeler of each antennule. Each olfactory seta consists of two segments, but the distal segment is not barbed as in a tactile seta (Fig. 100 B).

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10. Reproductive System in Prawn:

Prawns are dioecious, that is, the sexes are separate.

The males are distinguished from the females by the following external characters:

(2) Bases of the walking legs are more closely approximated in the males than in females;

(3) Second walking legs are stouter and covered with greater number of spines in the male than in females;

(4) Second pleopods, in the male, carry an additional stylet called appendix masculina which is absent in the female. .

(5) The paired gonopores are situated at the bases of the fifth walking legs in the male, whereas in a female the gonopores are situated at the bases of the third walking legs.

The male reproductive organs consist of:

(1) A pair of testes,

(2) A pair of vasa deferentia,

(3) A pair of seminal vesicles, opening to the exterior by

(4) The male gonopores.

The testes are soft elongated structures lying above the hepatopancreas but beneath the heart. Anteriorly, they extend as far as the renal sac and posteriorly as far as the first abdominal segment. The two testes are united at their anterior ends to form a common lobe, while posteriorly they remain free.

The cardio-pyloric strand, which binds the heart to the pyloric stomach, passes through the gap between the posterior parts of the two testes. A testis is composed of a large number of lobules held together by connective tissue. Spermatozoa are budded off from the inner wall of the hollow lobules. A mature sper­matozoon looks like an open umbrella.

A vas deferens arises from the outer side of each testis. It is a closely coiled tube at its commencement, which soon straightens up and extends backwards and downwards to the base of the fifth walking leg of the same side where it dilates to form a club-shaped seminal vesicle. The spermatozoa are stored within the seminal vesicle which finally opens into the male gonopore of the side.

The male gonopores are paired; each is a small opening covered by a tongue-shaped flap of integument and is situated at the base of the fifth walking leg. The spermatozoa are carried by the vasa deferentia into the seminal vesicles where they are stored in the form of compact pockets called spermatophores which are released through the gono­pores.

The female reproductive organs consist of:

(1) A pair of ovaries and

(2) A pair of oviducts opening to the exterior by

(3) The female gonopores.

The ovaries are situated on the dorsal surface of the hepato pancreas and just beneath the pericardial sinus of the heart. The shape and size of the ovaries vary according to the age of the individual and the season of the year.

In a fully grown female they extend anteriorly up to the renal sac and posteriorly up to the first abdominal segment. The ovaries are brown sickle- shaped bodies which are united at both the ends, leaving a gap in the middle for the passage of the cardio-pyloric strand. The ova (eggs) are formed in the ovaries. A mature ovum is a rounded cell with a large nucleus, and its cytoplasm is full of yolk granules.

During the breeding season, the ovaries increase in size to hold the mature eggs.

An oviduct arises from the outer side of each ovary. It is large and funnel-shaped at its commencement, and then gradu­ally becomes narrow and tubular. The oviducts are shorter in length -but broader than the vasa deferentia; they plunge verti­cally downwards to the female gonopores. The female gonopores are paired; each is a small opening situated on the inner side of the base of the third walking leg.

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11. Breeding and Life History of Prawn:

Prawns breed during the rainy season. Females carry fertilized eggs on the under surface of her abdomen, in the egg-carrying basket formed by the inter­locking of the appendices internae of the second, third, fourth, and fifth pleopods.

The fertilized eggs develop directly into young individuals, which unlike most other crustacea, resemble small edition of their parents. They cling to the setae on the swimmerets of their mother for some time, and then undergo moulting. Immediately following the moulting, the young grow in size. Ulti­mately they separate and live independently.

In the marine prawn, Penaeus, and in most other crustacea, the develop­ment is indirect. There is a zoea larva which undergoes metamorphosis and ultimately develops into an young individual resembling its own species.