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In this article we will discuss about Palaemon Malcolmsonii:- 1. Habit and Habitat of Palaemon Malcolmsonii 2. External Features of Palaemon Malcolmsonii 3. Appendages 4. Body Wall 5. Muscular System 6. Endophragmal Skeleton 7. Digestive System 8. Respiratory System 9. Blood Vascular System 10. Reproductive System 11. Fertilisation and Development 12. Endocrine System.
Contents:
- Habit and Habitat of Palaemon Malcolmsonii
- External Features of Palaemon Malcolmsonii
- Appendages of Palaemon Malcolmsonii
- Body Wall of Palaemon Malcolmsonii
- Muscular System of Palaemon Malcolmsonii
- Endophragmal Skeleton of Palaemon Malcolmsonii
- Digestive System of Palaemon Malcolmsonii
- Respiratory System of Palaemon Malcolmsonii
- Blood Vascular System of Palaemon Malcolmsonii
- Reproductive System of Palaemon Malcolmsonii
- Fertilisation and Development of Palaemon Malcolmsonii
- Endocrine System of Palaemon Malcolmsonii
1. Habit and Habitat of Palaemon Malcolmsonii:
Palaemon inhabits freshwater streams, rivers, lakes and ponds. It is a nocturnal creature and lies hidden at the bottom during the day and comes to the surface during night in search of food. It prefers slow moving clean water and feeds on algae, mosses and other weeds, small insects and the debris, i.e., omnivorous.
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It walks slowly at the bottom with the help of walking legs and swims actively at the surface with the help of swimmerets (pleopods). P. malcolmsonii breeds in May, June and July and the females carry eggs attached to their pleopods (abdominal appendages).
2. External Features of Palaemon Malcolmsonii:
(i) Shape and Size:
The body of Palaemon is elongated, bilaterally symmetrical. The size of the adult Palaemon varies considerably in different species. P. carcinus from Travancore measures 90 cm, P. malcolmsonii from Central Provinces and Chennai is generally 25 to 38 cm and P. lamarrei, another common species in lakes and ponds, is only 2.5 to 5 cm in length.
(ii) Colouration:
The young specimens are transparent, pale yellow or perfectly white in appearance, but the older specimens are differently tinted according to the species. The colour is generally pale blue or greenish with brown or orange-red patches and bands of different patterns. The preserved specimens assume deep orange-red colour on a white or yellow background.
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(iii) Division of Body:
The body of adult Palaemon Malcolmsonii is consisting of nineteen appendage bearing segments having two distinct regions an anterior rigid cephalothorax and a posterior movable abdomen.
(i) Cephalothorax:
The cephalothorax is somewhat cylindrical in shape and is un-jointed; no external segmentation is marked. It is formed by the union of head comprising five appendage- bearing segments and the thorax comprising eight segments.
(ii) Abdomen:
The abdomen, rounded dorsally but compressed laterally, is jointed, i.e., external segmentation is clearly marked and composed of six movable segments having a terminal conical piece, telson. The abdomen is often turned downwards. Each segment of the abdomen carries a pair of jointed appendages called pleopods or swimmerets.
(iv) Exoskeleton:
The entire body and the appendages are covered by a hard and chitinous cuticle which forms exoskeleton.
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The exoskeleton is hardened by deposit of salts of lime and is coloured with various pigments. The hard pieces of exoskeleton are called sclerites which are joined by softer cuticle forming arthrodial membrane. The cephalothorax consists of a head and thorax, the head is covered with a dorsal plate and thorax with a carapace.
The dorsal plate and carapace are completely fused to form a continuous dorsal shield. The dorsal shield is produced in front into a rostrum which is laterally compressed and is serrated. At the base of rostrum, on either side is an orbital notch which accommodates the stalk of compound eye.
The dorsal plate has two short spines on each side, they are an antennal and a hepatic spine. The carapace hangs down on the sides of the thorax as a branchiostegite or lateral plate which encloses the gill chamber. On the ventral side of head and thorax are chitinous sclerites called sterna. The cephalothorax is joined to the abdomen by a calcified hardened arthrodial membrane.
The dorsal shield covers the head and thorax dorsally and laterally. On the ventral side are sterna, they are an anterior most ophthalmic sternum, behind it an antennular sternum, followed by an antennal sternum or epistome to which a medium labrum is attached.
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The third and fourth segments have no sterna. The sterna of segments 5 to 13 have fused to form a floor. On the sides of segments 5 to 13 are 9 pairs of chitinous sclerites called epimera, one epimeron on each side of a segment.
In the abdomen each segment has a dorsal tergum or tergite which is broad and curved, a narrow ventral sternum or sternite, on each side is a pleuron formed from the tergum. On each side in the thorax and abdomen the pleuron is joined to an appendage by an epimeron which is a part of the pleuron.
Between the segments the two terga are joined by an inter-tergal arthrodial membrane, and two sterna by an inter-sternal arthrodial membrane. In the abdomen each segment articulates with the other laterally by pair of hinge-joints made of a ball and socket which permit movement of a segment on another in a vertical plane, but there is no movement from side to side.
(v) External Apertures of Palaemon Malcolmsonii:
There are following external apertures in Palaemon Malcolmsonii:
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(a) Mouth:
It is a mid-ventral slit-like aperture lying at the anterior end of the cephalothorax.
(b) Anus:
It is a longitudinal slit-like aperture lying at the ventral end of the base of telson.
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(c) Renal Apertures:
These are paired; each situated on a raised papilla at the inner surface of the coxa of antennae.
(d) Openings of Statocysts:
These are paired; each situated in a depression at the dorsal surface on the basal podomere (precoxa) of antennules.
(e) Female Genital Apertures:
These are paired; each situated at the inner surface of the coxae of third pair of walking legs.
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(f) Male Genital Apertures:
These are also paired; each situated at the inner surface of the coxae of fifth pair of walking legs.
3. Appendages of Palaemon Malcolmsonii:
From the ventrolateral margin of each segment arises a pair of appendages or limbs. All the appendages are biramous (L., bi = two + ramus = branch), except the first pair which are uniramous (L., uni = one + ramus = branch). Each appendage has a basal stalk or protopodite made of two joints or podomeres, a lower coxapodite or coxa and an upper basopodite or basis.
From the protopodite arise two rami, an inner endopodite and an outer exopodite, each of which may be composed of one to several podomeres. The first appendage is not comparable in detail with the others, it is uniramous.
Each appendage has a cuticular exoskeleton which is divided in most into tube-like segments or podomeres connected to one another by soft articular or arthrodial membrane, thus, a joint is created at each junction, these joints enable the podomeres of appendages to move. In each podomere are two protractor and two retractor muscles attached to the inner surface of cuticle.
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Flexion and extension of appendages are brought about by the muscles, with the muscles and cuticle acting together as a lever system.
This co-ordination of muscular and skeletal systems for locomotion is essentially the same as in vertebrates, the only difference is that in arthropods muscles are attached to the inner surface of an exoskeleton, whereas in vertebrates the muscles are attached to the outer surface of an endoskeleton.
Generally all the appendages are one of the following two types:
(a) Stenopodium is a slender limb, its endopodite and exopodite are set on a common stalk, the protopodite.
(b) Phyllopodium is broad and leaf-like with thinner cuticle, the endopodite and exopodite do not form a fork.
In Palaemon, however, as referred to, all the appendages are built on a biramous plan (except the first) but they are modified variously; it is because of their different functions.
A. Cephalic Appendages:
Of the five pairs of cephalic appendages, first two, i.e., antennules and antennae are pre-oral, while mandibles, maxillulae and maxillae are post-oral.
1. Antennules:
The antennules are situated, one on either side, immediately below the bases of eye-stalks.
Antennules are uniramous. Each antennule consists of a protopodite having three podomeres, precoxa, coxa and basis and a pair of slender many jointed feelers. The precoxa is very large and bears a depression, containing the opening of statocyst on its dorsal side and a spiny lobe called stylocerite on its outer margin. The coxa is short and cylindrical.
The basis is longer than coxa. From the basis arises a pair of long, jointed feelers which are probably not homologous with the exopodite and endopodite of a typical appendage. The outer feeler is divided into two unequal branches. Antennule is tactile and its statocyst is for equilibration. Antennules are homologous with the antennae of other mandibulate classes.
2. Antennae:
The antennae are situated, one on either side, just below and behind the antennules. The protopodite is considerably swollen due to the presence of the excretory organ within it, which opens by minute renal aperture on the inner margin of coxa.
The exopodite is broad leaf-like plate, the squama, while the endopodite is long many jointed feeler. The squama bears setae along its inner and distal margin and probably serves the function of a balancer during swimming. The antennae are tactile, excretory and balancing in function.
3. Mandibles:
The mandibles are short but stout appendages, lying one on each side of the mouth. The greater part of each mandible consists of the coxa which is differentiated into a proximal spoon-shaped apophysis and a solid distal head. The head consists of two parts a molar process with five or six dental plates and a flat plate-like incisor process with three teeth.
The outer margin of the head carries a small three jointed mandibular palp. The proximal segment of the mandibular palp represents the basis of the protopodite, while the two distal segments represent the endopodite. The exopodite is absent. The mandibles are masticatory in function.
4. Maxillulae:
The maxillulae are small, thin and leaf-like appendages. Coxa and basis are broad, and project inwards as jaws or gnathobases (Gr., gnathos = jaw) fringed with bristles. Endopodite is curved and bifurcated terminally, it has small hooks. Exopodite is absent. The maxillulae are used for passing food to the mouth.
5. Maxillae:
The maxillae are also thin and leaf-like appendages. The coxa is very small and incompletely divided into two, while the basis is larger forming a bifurcated gnathobase with stiff setae at its inner end.
The endopodite is small, while the exopodite forms a large fan-shaped scaphognathite or baler with plumose bristles along its border. The scaphognathite projects into a gill-chamber causing a current of water on the gills. Maxillae have both feeding and respiratory function.
B. Thoracic Appendages:
The thoracic appendages consist of the anterior three pairs of maxillipedes (Gr., maxilla = jaw + podos = foot) or foot jaws and posterior five pairs of walking legs or paraeopods.
1. First Maxillipedes:
The first maxillipedes or foot jaws are thin and tend to be phyllopodium. Coxa and basis form flat leaf-like gnathobases or endites by its inner borders. On its outer side coxa bears a bilobed leaf-like epipodite which acts as a gill. The endopodite is small and unsegmented.
The exopodite is longer and bears a thin plate-like expansion from its base. The margin of exopodite and endopodite are fringed with setae. First maxillipedes hold the food and also server the function of tactile and respiratory organs.
2. Second Maxillipedes:
The coxa is small with an epipodite and a gill on its outer border. The exopodite is long and slender. The endopodite arises from the basis and consists of five podomeres called ischium, merus, carpus, propodus and dactylus. Of these propodus and dactylus lie parallel and form a cutting plate with bristles. Second maxillipedes are tactile, hold food and are respiratory.
3. Third Maxillipedes:
The third maxillipedes are distinctly leg-like in appearance. The coxa bears a small epipodite on its outer side. The basis is small and supports a slender unjointed exopodite and an elongated endopodite consisting of three podomeres.
In the endopodite the first podomere represents fused ischium and merus, second is the carpus and third represents fused propodus and dactylus. The exopodite is thickly covered with setae all along its length. Third maxillipedes are tactile, hold food and are respiratory.
Walking legs:
The five pairs of walking legs of Palaemon differ from the maxillipedes in the absence of the exopodites and epipodites. A typical walking leg, like the fourth, consists of a two jointed protopodite and a five jointed endopodite. The protopodite consists of a short ring-shaped coxa and a triangular basis.
The endopodite consists of five podomeres, viz., ischium, merus, carpus, propodus, and dactylus.
All the seven podomeres are arranged in a linear series and are movably hinged together. In the first and second pairs of walking legs the propodus is prolonged distally beyond its articulation with the dactylus, so that the two podomeres work one against the other like the blades of a pair of forceps forming a chela or pincers with sharp terminal claws.
In fact, dactylus is movably articulated to propodus. Such legs are called chelate legs or chelipeds, and are used to catch the prey and transfer it to the mouth and also serve as organs of offense and defence. The second chelate legs in male are larger and more powerful than in the female.
The third, fourth and fifth pairs of walking legs are typical and non-chelate. In female, each of the third leg bears a crescentic female genital aperture on the inner side of its coxa, while in male, each of the fifth leg bears a slit-like male genital aperture on its side of the arthrodial membrane connecting the leg and thorax.
C. Abdominal Appendages:
The six pairs of abdominal appendages are known as pleopods or swimmerets. Abdominal appendages are simple and typical biramous type. The third abdominal appendage may be taken as a typical type.
Third Abdominal Appendages:
Protopodite consists of only two podomeres, a short ring-like proximal coxa and a long cylindrical distal basis. The basis bears flattened leaf-like smaller endopodite and larger exopodite.
Closely applied against the inner margin of the endopodite, there is a short and slender, slightly curved rod called appendix interna with a knob-like head, bearing many hook-like processes. In the female, second, third, fourth and fifth abdominal appendages serve to carry eggs in the breeding season.
The appendix interna of each of these appendages becomes interlocked with its fellow of the opposite side, thus, leading to the formation of a series of bridges on the ventral side of the abdomen which serve to carry the eggs. The remaining five abdominal appendages resemble this typical appendage in all essential features but there are small differences which are given below.
First Abdominal Appendages:
In the first abdominal appendages, the endopodite is greatly reduced and the appendix interna is totally absent.
Second Abdominal Appendages of Male:
In the second abdominal appendages of male, the appendix interna gives off on its inner side an additional process called appendix masculina lying in between the appendix interna and endopodite.
Uropod:
The sixth or uncalcified the last pair of abdominal appendages are very large and form broad flat plates and lie one on either side of the telson. These appendages are called uropods or tail feet. In the uropod, the coxa and basis are fused together to form a sympod. The exopodite and endopodite are oval and oar-shaped.
The exopodite is broad and large and is divided into two unequal parts by a transverse suture. The endopodite is slightly smaller and is undivided. The margins of exopodite and endopodite, except the outer border of the exopodite, are fringed with long plumose setae. The two uropods with the telson form a fan-like tail fin which causes the strong and rapid backward spring of prawn in swimming.
4. Body Wall of Palaemon Malcolmsonii:
The body wall of Palaemon Malcolmsonii (Fig. 71.8) consists of an outer thick layer of cuticle, a middle single-layered epidermis and an inner dermis.
(i) Cuticle:
The body is covered with a thick two- layered cuticle, the outer layer is a thin non-chitinous epicuticle and an inner layer is a thick laminated endocuticle. The epicuticle is made of an outer lipoid layer and an inner protein layer. It is produced into spines and setae of different forms and bears, at places, the fringes of setae.
The endocuticle contains layers of chitin and has three successive layers—the outermost layer is a thin pigmented layer, below it is a thick calcified layer and under it is an uncalcified layer. The pigmented layer is dark due to protein deposits and it becomes hardened by a process of tanning or sclerotisation. In the calcified layer are deposists of carbonates and phosphates of calcium which make the integument hard.
The chitin is an acetate of a polysaccharide containing glycosamine. The cuticle is relatively impermeable, except where it is thin and allows the passage of gases or absorption of water. The cuticle is cast off once a year in the adult, this is called moulting or ecdysis.
(ii) Epidermis:
Below the cuticle is a thick epidermis. It comprises a layer of glandular columnar epithelial cells resting on a thin basement membrane.
(iii) Dermis:
Below the epidermis is dermis. The dermis is made of loose connective tissue containing the blood lacunae. Situated within the connective tissue layer are a number of tegumental glands. Each gland consists of a cluster of secreting cells with nuclei placed at their bases.
Each gland-cell has a capillary canal and the canals of all the gland-cells join together in the centre of the gland to open into a long narrow cuticular duct leading to the exterior on the surface of the epicuticle. In the connective tissue are also found chromatophores. The epicuticle is non-chitinous and is secreted by the tegumental glands, while endocuticle is chitinous and is secreted by the epidermis.
5. Muscular System of Palaemon Malcolmsonii:
The muscles are almost all entirely striped, they are all longitudinal, except in the heart and arteries where they are circular.
The muscles are concerned with extension and flexion of abdomen and appendages. Seven pairs of extensor muscles arise from the six abdominal segments and telson, they run dorsally and are inserted on the lateral wall of the thorax, when these muscles contract they extend arid straighten the abdomen.
There are five paires of abdominal flexor muscles arising from the first five abdominal segments, they lie ventrally and are very strong, they are inserted on the sterna, epidermis and thoracic wall. When flexor muscles contract they bend the abdomen. The appendages have hollow, tubular podomeres, two adjacent prodomeres being connected by a hinge joint having a soft arthrodial membrane which permits movement.
Each podomere has two extensor and two flexor muscles joined to preceding and succeeding podomeres, the flexor muscles bend the podomere and extensor muscles straighten it. In Crustacea each muscle has only two to five nerves, they are motor and sensory nerves, besides which there is an inhibitory nerve which inhibits or checks muscles contraction.
6. Endophragmal Skeleton of Palaemon Malcolmsonii:
An internal skeleton is formed by ingrowths of the cuticle called apodemes which serve for insertion of muscles. In prawn apodemes unite to form an endophragmal skeleton. It consists of rods lying between epimera and sterna of cephalothorax. It is best developed between segments 3/ 4 and segments 11/12 and 12/13.
Between third and fourth segments are two large apodemesjoined by a transverse fibrous strand, together they form a cephalic apodeme for attachment of mandibular muscles.
In succeeding segments on each side there are two adjacent apodemes, an endopleurite arising from the epimeron, and an endosternite arising from the sternum. Between segments 11 and 12, and 12 and 13 there arises on each side from the endosternite a Y-shaped rod, its inner arm is mesophragm and outer arm! is paraphragm, they are for attachment of flexor abdominal muscles.
7. Digestive System of Palaemon Malcolmsonii:
In Palaemon Malcolmsonii, the digestive system is composed of alimentary canal and hepatopancreas.
Alimentary Canal of Palaemon Malcolmsonii:
The alimentary canal consists of foregut, midgut and hindgut. The foregut comprises the mouth, buccal cavity, oesophagus and stomach. The foregut and hindgut have an internal lining of cuticle (intima) but the midgut has a soft lining of endoderm.
(i) Mouth:
The mouth is a large slit-like aperture situated mid-ventrally below the anterior end of head between the third and fourth segments. It is bounded in front by a shield-shaped labrum, on the sides by the incisor processes of the mandibles, and behind by the labium which is cleft to form two lobes or paragnatha.
(ii) Buccal Cavity:
The mouth leads into a short buccal cavity. It is anteroposteriorly compressed and has a thick chitinous lining which is thrown into irregular folds. The molar processes of the mandibles project into the buccal cavity from two sides to crush the food between them.
(iii) Oesophagus:
The buccal cavity leads into short but broad oesophagus running almost vertically upwards from the buccal cavity to the stomach.
Internally the thick muscular wall of the oesophagus is thrown into four prominent longitudinal folds projecting into the lumen. Of these folds, the anterior is short but the posterior and lateral folds are longer and more prominent. Each of the lateral folds being subdivided into two smaller unequal folds.
(iv) Stomach:
The oesophagus leads into a spacious chamber, the stomach, which occupies most of the cephalothoracic cavity. It is surrounded laterally, ventrally and posteriorly by the hepatopancreas.
The stomach is divided into two parts:
(a) A large anterior bag-like cardiac stomach, and
(b) A very small posterior pyloric stomach. Cardiac and pyloric stomach are separated by a number of valves.
(a) Cardiac Stomach:
The cardiac stomach is lined internally with delicate cuticle or intima. The inner lining of intima is produced into a very large number of inconspicuous longitudinal folds covered by minute bristles. The wall of the stomach is supported by some cuticular plates embedded in its internal lining.
The anterior wall of the oesophageal opening is covered by a thin circular cuticular plate and there is a similar lanceolate plate on the anterior part of the roof of the cardiac stomach the cardiac stomach.
A large triangular plate is embedded in the middle of the floor of cardiac stomach, it is called the hastate plate because it looks like the head of a spear. The upper surface of hastate plate is covered with a thick growth of delicate setae and bears a distinct median ridge with gradually sloping sides.
The posterior triangular part of the plate is depressed, thickly fringed with setae all along its posterior border and forms the anterior valve of the cardio-pyloric aperture. The lateral sides of the hastate plate are supported beneath by a pair of longitudinal cuticular supporting rods.
A narrow lateral groove runs along either lateral border of the hastate plate. The floor of each lateral groove is covered by a cuticular plate which is like an open drainpipe and is called the groove plate. Each lateral groove is bounded on its inner side by the supporting rod and on the outer side by a long cuticular ridged plate.
The inner border of each ridged plate is forged all along with a row of closely set delicate bristles, forming a long comb-like structure, therefore, called a combed plate. The bristles of combed plate cover the lateral groove and partially overlap the lateral margins of the hastate plate.
The fine bristles of the comb plate constantly keep moving to and fro over the outer margin of the hastate plate. The two combed plates united anteriorly, and completely enclose the hastate plate except that their incurved posterior ends remain separated by the cardio-pyloric aperture. Just outside the combed plates, the wall of cardiac stomach is folded on each lateral side to form two bluish black lateral longitudinal folds.
These folds are low in front but gradually increase in height behind, but posteriorly the high walls of these folds bend inwards over the cardio-pyloric aperture. These lateral longitudinal folds are also known as guiding ridges because they guide the food towards the cardio-pyloric aperture.
The cardio-pyloric aperture is narrow X-shaped and is guarded by four valves. The anterior valve is formed by the low-lying posterior triangular area of hastate plate, the lateral valves by the large flap-like posterior ends of guiding ridges and posterior valve by a semi-lunar fold of posterior wall of cardiac stomach. The cardio-pyloric aperture leads into the pyloric stomach.
(b) Pyloric Stomach:
The pyloric stomach is a very small and narrow chamber lying below the posterior end of the cardiac stomach.
Its walls are thick and muscular, they divide the lumen of the pyloric stomach into a small dorsal chamber and a large ventral chamber, the two communicating with each other by a narrow vertical slit-like aperture. The floor of the chamber is raised into a median longitudinal ridge so as to divide its cavity into two lateral compartments.
The floor of ventral chamber is covered by a A-shaped filter plate, each side of which is rectangular in outline and bearing a series of alternating longitudinal ridges and grooves.
The ridges bear rows of horizontal comb-like bristles which form a felt-like covering over the grooves. The lateral walls of the ventral chamber are also lined with thick cuticle covered with closely set bristles, which together with the filter plate form an efficient filter or strainer.
The A-shaped plate is, therefore, called the pyloric filter plate which allows only liquid food to pass through it. The two openings of the duct of hepatopancreas lie at the posterior end of the ventral chamber just behind the filtering apparatus. The dorsal chamber gives off a short blind caecum before it becomes continuous with the midgut.
Midgut:
The midgut is a long, narrow, straight and slender tube running back along the median line above the mass of the ventral abdominal muscles up to the sixth abdominal segment. The internal lining of midgut is formed by epithelium which in the posterior part is thrown into many longitudinal folds, thus, greatly reducing its lumen.
Hindgut:
The hindgut extends from the posterior end of the midgut to the anus and forms the shortest portion of the alimentary canal. Anteriorly it is swollen into a thick muscular sac called the rectum, while its posterior part is narrow and tubular which opens to the exterior through the anus.
The rectum bears several thick longitudinal folds which project into the lumen of the hindgut. The anus is a longitudinal slit situated on the raised papillae and it is provided with a sphincter muscle and a number of radiating muscle fibres.
Hepatopancreas:
The hepatopancreas or the so called liver is a large bilobed, compact, orange-coloured glandular mass occupying a considerable area of cephalothoracic cavity. It lies immediately behind the cardiac stomach but a part of it lies around the floor and sides of the stomach. Dorsally it is covered over by the reproductive organs and the heart, while ventrally it overlies the nerve cord.
Developmentally it is formed from one pair of lateral tubular outgrowths, the hepatic caeca, of the midgut. The hepatopancreas of adult consists of numerous glandular tubules branching in a racemose manner, the branches are held together by the connective tissue as to form a compact mass.
The wall of the tubules is made of a single layer of columnar epithelial cells having granular cells, ferment cells, hepatic cells and basal or replacing cells.
The epithelial cells rest on a basement membrane. The canals of the numerous tubules unite together at intervals forming larger and larger canals, finally forming the two large hepatopancreatic ducts which open into the ventral chamber of pyloric stomach just behind the pyloric filter plate, one on each side.
The hepatopancreas combines within itself the functions of the pancreas, liver and small intestine of higher animals. Like the pancreas it secretes digestive ferments which are capable of digesting carbohydrates, proteins and fats. Like the midgut it absorbs digested food material. It also serves as an important storage organ for glycogen, fat and calcium like the liver.
Food and Feeding of Palaemon Malcolmsonii:
The food of Palaemon Malcolmsonii comprises algae, moss and other aquatic weeds, and occasionally some small aquatic insects and debris from the bottom of the water.
The chelate legs pick up the food and take it to the mouth, the second and third pairs of maxillipedes hold the food in position for the mandibles, to cut up the food into small pieces by their incisor processes, then food is transferred to the mouth. In the mouth cavity the molar processes of mandibles crush the food which goes to the cardiac stomach through the oesophagus.
Digestion, Absorption and Egestion of Palaemon Malcolmsonii:
The smaller particles of the food from the cardiac stomach are passed on to the pyloric stomach. The digestive secretion of the hepatopancreas is poured into the pyloric stomach through the hepatopancreatic ducts. Secretion reaches the cardiac stomach and mixes with the food. By the contraction and expansion of cardiac stomach the food is churned and also digested by the digestive ferments.
When the food passes over the hastate plate the moving bristles of the combed plates cut it into fine particles. The food in the semi-liquid and semi-digested form is filtered through the bristles of the combed plates, into the lateral grooves below, whence it is carried into the ventral chamber of the pyloric stomach through the cardio-pyloric aperture.
The digested and liquefied food is filtered through the pyloric filter and it again enters the hepatopancreas through the hepatopancreatic ducts and then becomes absorbed.
The hepatopancreas serves for digestion as well as for absorption. The residual food material, consisting of undigested particles, ascends up the dorsal pyloric chamber and from there enters the midgut. The remaining digested food is absorbed in the midgut. The undigested faecal matter passes on to the posterior side of the hindgut and ultimately expelled through the anus.
8. Respiratory System of Palaemon Malcolmsonii:
In Palaemon Malcolmsonii, the respiratory organs are well developed and consist of:
1. Branchiostegites or gill covers
2. Three pairs of epipodites
3. Eight pairs of gills or branchiae
1. Branchiostegites:
Each gill chamber is enclosed between the branchiostegite or carapace on the outer side and the thoracic wall on the inner side.
The gill chambers open on all sides except dorsally. The inner linings of branchiostegites are thin, membranous and highly vascular containing blood lacunae. These are constantly bathed in freshwater, thus, forming the respiratory surfaces which absorb oxygen dissolved in water and give out carbon dioxide.
2. Epipodites:
There are three pairs of simple, leaf-like and highly vascular epipodites which are the outgrowths of the coxal podomeres of three maxillipedes. The epipodites lie in the anterior part of the gill-chamber below the scaphognathite. The epipodites serve as respiratory organs like the primitive gills.
3. Gills:
There are eight gills or branchiae lying inside each gill-chamber but only seven of them are exposed at once on the removal of gill-cover as the eighth lies hidden beneath the dorsal part of the second gill.
Types of gills:
Gills are named according to their place of origin and attachment.
There are three kinds of gills which are as follows:
(i) Podobranch or foot-gill:
When a gill is attached to the coxa of an appendage, it is known as podobranch or foot-gill. There is one podobranch attached to the coxa of each second maxillipede.
(ii) Arthrobranch or joint-gill:
When a gill is attached to the arthrodial membrane connecting the appendage to the thorax, it is called an arthrobranch or joint-gill. There are two arthrobranchs attached to the arthrodial membrane of third maxillipede.
(iii) Pleurobranch or side-gill:
When a gill is attached to the lateral wall of the segment having the limb, it is called a pleurobranch or side-gill. There are five pleurobranchs attached to the lateral walls of the thoracic segments bearing five walking legs.
Branchial formula:
The number and disposition of respiratory organs of each gill-chamber can be represented in the form of branchial formula as shown in the following table:
Branchial Formula of Palaemon:
Structure of gill:
The gills are more or less crescentic in shape with a gradual increase in size anteroposteriorly, i.e., each gill is larger than one in front of it. The dorsal end of each gill projects into the antero-dorsal depression of the gill chamber, while its ventral end lies close to the lower margin of the gill cover.
Each gill is attached in the middle of its length to the thoracic wall by a small connection called the gill-root. Nerves and blood vessels enter or leave the gills through the gill-roots.
In Palaemon Malcolmsonii, all the gills are phyllobranch, i.e., each gill consists of two rows of leaf-like rhomboidal gill- plates arranged like the leaves of book and lie at right angles to the long axis or the base of the gill.
The gill plates are largest in the middle but become gradually smaller towards the two ends of the gill. Between the two rows of gill- plates runs a deep median longitudinal groove which opens into the gill- chamber at both the dorsal and ventral ends of the gill.
Histological structure of gill:
A transverse section of a gill reveals that the axis or base is roughly triangular in outline and consists of central core of connective tissue enclosed by a layer of epidermis which is externally protected by a thin cuticle. A gill plate is made up of single layer of cells with cuticle on both sides. The cells are of two types, viz., pigmented and transparent, alternating with each other.
Vascular supply of the gills:
Three longitudinal blood channels run through each gill base. There are two lateral longitudinal channels, running along the lateral margins, one on each side. The third is a median longitudinal channel, which runs through the apex of gill base below the median groove of the gill.
The two lateral longitudinal channels are connected with each other by a series of small transverse connectives which present a ladder-like appearance. At the place of attachment of each gill-plate on the axis, a slender marginal channel is given off from each lateral longitudinal channel which runs all along the margin of its gill-plate and opens into the median longitudinal channel.
Course of blood in a gill:
Non- aerated or impurified blood from the body is brought to the gill by an afferent branchial channel which opens into a transverse connective lying immediately against the root of the gill.
The blood then flows into both the lateral longitudinal channels and passes through the marginal channels and reaches the median longitudinal channel. During this course the blood gets oxygenated. From the median longitudinal channel of each gill all the oxygenated blood is carried by an efferent branchial channel to the pericardial sinus and heart.
Mechanism of Respiration:
The scaphognathite of each maxilla vibrates constantly causing a current of water in the gill chamber.
The freshwater enters the gill chamber from behind in the form of a current along the posterior and ventral margins of the gill cover. The water then flows over the gills and epipodites and reaches the antero-dorsal depression of the gill chamber wherefrom it is expelled out at the anterior end by the baling action of the scaphognathite.
The gills and epipodites are richly supplied with blood and exchange of gases takes place on the surface of gills and epipodites. The oxygen dissolved in water is taken in by blood and carbon dioxide diffuses out in the water. The action of scaphognathite is supplemented by the movements of the exopodites of maxillipedes.
9. Blood Vascular System of Palaemon Malcolmsonii:
In Palaemon Malcolmsonii, the blood vascular system is of the open type.
It comprises:
1. Pericardium
2. Heart
3. Arteries
4. Sinuses or blood-lacunae
5. blood channels
There are no veins and capillaries in Palaemon.
1. Pericardium or Pericardial Sinus:
It is a wide spacious sinus lying just below the dorsal wall of the thorax and above the reproductive organs and hepatopancreas. The floor of pericardium is in the form of a thin horizontal pericardial septum which is attached in front and behind to the dorsal body wall and on the two sides to the lateral walls of the thorax.
The pericardium is incompletely filled with blood which it gets from the channels and passes into the heart, enclosed within the pericardium.
2. Heart:
The heart is a muscular triangular structure with its apex in front and base behind. It is enclosed in the pericardium in the median dorsal part of thorax dorsally to the alimentary canal. A median longitudinal cardio-pyloric strand runs from the apex of the heart to the pyloric stomach and two lateral strands, each extending from the lateral angle of the heart to the body wall.
These strands keep the heart in position inside the pericardium. The thick and muscular wall of the heart is perforated by five pairs of apertures or ostia (L., ostium = a door). Each ostium is a small, slit-like opening and its lips act as valves allowing blood to flow only in one direction, i.e., from pericardial sinus into the heart and not vice versa.
The ostia are so distributed that the first pair lies mid-dorsally, second pair mid-ventrally, the third pair posteriorly, the fourth pair antero-laterally and the fifth pair postero-laterally. In a transverse section, the heart appears like a thick spongy meshwork of muscle fibres, the small cavities of which represent the cavity of the heart. The cavity of the heart is not continuous but is traversed by numerous muscle fibres.
3. Arteries:
The oxygenated blood flows from the pericardial sinus into the heart. When the heart contracts, it pumps out all its blood into narrow tubes which are thick, strong and muscular and are known arteries.
The principal arteries of Palaemon Malcolmsonii are as follows:
(i) Median Ophthalmic or Cephalic Artery:
It is a single slender artery, which arises from the apex of the heart. It runs forward just below the carapace along the mid-dorsal line of the renal sac between the mandibular muscles. Anteriorly it reaches the roof of the oesophagus where it joins the two antennary arteries. It supplies blood to the oesophagus, cardiac stomach and the head.
(ii) Antennary Arteries:
A pair of antennary arteries arise from the apex of the heart, one on each side of the median ophthalmic. Each antennary artery runs obliquely forwards passing along the outer side of the mandibular muscle.
It gives off three branches, viz.:
(a) A pericardial branch to the pericardium,
(b) A gastric branch to the cardiac stomach and
(c) A mandibular branch to the mandibular muscles. Each antennary artery passes forwards and again divides into a dorsal and ventral branch.
The dorsal branch divides into two sub-branches, of which the outer, the optic artery supplies the eye, while the inner meets its fellow of the opposite side and the median ophthalmic artery to form a loop-like artery, the circulus cephalicus which gives off a pair of rostral arteries to the rostrum.
The ventral branch also divides into two sub-branches, one of which supplies the antennule, the antennulary artery and the other re-divides into two, supplying the renal organ, the renal artery and antenna, the antennal artery.
(iii) Hepatic Arteries:
A pair of hepatic or hepatopancreatic arteries arise from the heart of the ventro-lateral sides, one on each side just behind the antennary. Each of them soon after its origin plunges downwards into the hepatopancreas and ramifies within it.
(iv) Median Posterior Artery:
From the middle of the postero-ventral surface of the heart arises a short stout median artery which immediately bifurcates into two branches a slender supra-intestinal artery and a stout sternal artery.
(a) Supra-intestinal artery:
The supra-intestinal artery or dorsal abdominal artery passes straight backwards along the dorsal surface of the intestine extending up to the hindgut, where it divides into two branches, one on each side of the hindgut. It supplies blood to the intestine and the dorsal muscles of the abdomen.
(b) Sternal artery:
The sternal artery is the stoutest artery in the blood vascular system of Palaemon. It runs obliquely downwards passing either to the left or to the right of the midgut.
It then pierces through an aperture in the middle of the ventral thoracic ganglionic mass to reach the ventral side where it divides into two branches:
(i) Ventral thoracic artery which is larger and runs forwards and supplies blood to the sternal region of the thorax, maxillae, maxillulae, maxillipedes and first three pairs of walking legs,
(ii) Ventral abdominal artery which runs backwards and supplies blood to the ventral region of the abdomen, fourth and fifth walking legs, six pairs of abdominal appendages and the hindgut.
4. Blood Sinuses or Lacunae:
The arteries repeatedly branch into capillaries in the various organs of the body.
The veins are entirely absent in Palaemon, therefore, arteries open into the blood sinuses or lacunae of the haemocoel. All the sinuses of the body eventually meet into a pair of elongated and ill-defined ventral sinuses which are situated below the hepatopancreas and the flexor muscles of the thorax. The ventral sinuses communicate with each other at several places.
5. Blood Channels:
From each ventral sinus blood is carried to the gill of that side by means of six afferent branchial channels. As there are two ventral sinuses, the blood channels of both the sides are six pairs. These channels are lacunar in nature and lie just on the inner side of the lateral thoracic wall and innervate the gills through the gill-roots.
The first channel supplies blood to the two arthrobranchs and podobranch of the second maxillipede and the remaining five channels supply blood to the pleurobranchs. The aerated blood from the gills of each side returns to the pericardial sinus through another series of six efferent branchial channels which also leave the gills through their gill-roots.
Course of Circulation of Blood:
Due to the rhythmic contraction of the heart, the blood is pumped out of the heart into the arteries which carry the aerated blood to all the parts of the body. The course of circulation of blood in the body of Palaemon can be diagrammatically represented by the Fig. 71.27.
Blood:
In Palaemon Malcolmsonii, the blood is a colourless, thin, watery fluid containing floating colourless leucocytes or the white amoeboid corpuscles, which are phagocytic in nature. The respiratory pigment is haemocyanin which is dissolved in the plasma of the blood. Haemocyanin is a compound of copper and protein. When oxidised it is bright blue, but is colourless when de-oxidised. The blood has the power of coagulation.
10. Nervous System of Palaemon Malcolmsonii:
The nervous system of Palaemon consists of:
1. Central nervous system
2. Peripheral nervous system
3. Sympathetic nervous system
I. Central Nervous System:
Central nervous system consists of:
1. Brain or a pair of supra-oesophageal ganglia,
2. A pair of circumoesophageal commissures,
3. Ventral thoracic ganglionic mass, and
4. A ventral nerve cord.
1. Brain or Supra – Oesophageal Ganglia:
The brain or supra-oesophageal ganglion is a bilobed structure which lies at the base of rostrum, anterior to the oesophagus.
It is embedded in a thick mass of fat. It is formed by the fusion of several ganglia as it appears from the fact that several nerves arise from it to innervate the eyes, antennules, antennae and labrum etc. However, from segmentation point of view it is supposed to be formed of protocerebrum, paired optic ganglia, mesocerebrum and metacerebrum.
The following nerves originate from the brain:
(i) Antennulary nerves:
A pair of antennulary nerves arise from below the origin of optic nerves. Each nerve enters the antennule of its side into which it sends a statocystic branch to statocyst.
(ii) Optic nerves:
A pair of stout optic nerves arise from the dorsal surface of the brain, one on each side, to supply the compound eye of its side.
(iii) Ophthalmic nerves:
A pair of ophthalmic nerves arise from the brain, one on each side, and supply the ocular muscles in the eye-stalks.
(iv) Antennary nerves:
A pair of stout antennary nerves originate from the ventral surface of the brain. Each nerve divides into two branches, the outer innervating the squama and the inner innervating the feeler of antenna.
(v) Tegumental nerves:
A pair of slender tegumental nerves arise just behind the origin of antennary nerves. These nerves innervate the labrum.
2. Circumoesophageal Commissure:
These are two stout nerves, which arise from the posterior end of brain and run backwards and downwards around the oesophagus. It unites ventrally with the sub-oesophageal ganglia which form the indistinguishable anterior part of the ventral thoracic ganglionic mass.
Each commissure bears a small commissural ganglion near its anterior end, and gives off small nerve to the mandible of its side. The two circumoesophageal commissures are connected with each other by a slender transverse commissure near the posterior end.
3. Ventral Thoracic Ganglionic Mass:
The ventral thoracic ganglionic mass is an elongated oval structure situated immediately above the thoracic sternal plates in the mid-ventral line. It is a composite mass which is formed as a result of fusion of the eleven pairs of ganglia. It gives off eleven pairs of nerves on the lateral sides. The first three pairs are of cephalic nerves, supplying the mandibles, maxillulae and the maxillae respectively.
The last eight pairs are of the thoracic nerves, of which the first three pairs give off branches to the three pairs of maxillipedes respectively and the remaining five pairs supply the five pairs of walking legs. Each nerve to a leg becomes bifurcated before entering the leg.
4. Ventral Nerve Cord:
The ventral thoracic ganglionic mass is continued posteriorly into the ventral or abdominal nerve cord. It runs posteriorly along the mid-ventral line of the abdomen up to the last segment. In each abdominal segment there is an abdominal ganglion. The ventral nerve cord and the abdominal ganglia are double.
Each of the first five abdominal ganglia gives off three pairs of nerves in its segment:
(i) A pair of pedal nerves supplying the pleopods,
(ii) A pair of nerves to the extensor muscles of its segment, and
(iii) A pair of nerves to the flexor muscles of the succeeding segment.
The last or sixth abdominal ganglion or stellate ganglion is comparatively large and is formed by the fusion of paired ganglion of the sixth segment with a number of post-abdominal ganglia. This ganglion supplies two pairs of nerves to the flexor muscles of the sixth segment, two pairs of nerves to the uropods, two pairs of nerves to the telson, and a single median nerve to the rectum and hindgut.
II. Peripheral Nervous System:
The various nerves originating from the central nervous system to innervate the different parts of the body constitute the peripheral nervous system.
III. Sympathetic Nervous System:
The sympathetic or visceral nervous system is represented by a few small ganglia and nerves. A small nerve arising from the posterior part of the brain, runs on the roof of the cardiac stomach and bears two visceral or oesophageal ganglia, one anterior and another posterior.
The anterior visceral ganglion is joined with the two commissural ganglia by a pair of connectives. The posterior visceral ganglion is free and gives off two pair of nerves to the muscles of the wall of oesophagus and the cardiac stomach.
11. Reproductive System of Palaemon Malcolmsonii:
Sexual Dimorphism:
Palaemon Malcolmsonii is dioecious, i.e., sexes are separate and the sexual dimorphism is well marked.
The males and females can be distinguished from each other by a number of external characters which are as follows:
1. The males are larger in size than the females.
2. In males, the bases of the thoracic legs are more closely approximated than in the females.
3. Males possess narrower abdomen than those of females.
4. In the males, the second pair of chelate legs are more elongated and more profusely covered with spines and setae than in the females.
5. In the males, each second pleopod bears an additional process, the appendix masculina which lies between the appendix interna and the endopodite.
6. In the males, the epimera of the abdominal segments are smaller than in the females, where they are bigger in size for carrying eggs.
7. In the males, the paired genital apertures are situated on the coxae of the fifth pair of walking legs, while in the females the paired genital apertures are situated on the raised papillae on the coxae of the third pair of walking legs.
Reproductive Organs of Palaemon Malcolmsonii:
Reproductive organs consist of gonads which are similar in shape, size, position, and general disposition in both the sexes. They are situated in the posterior region of the thorax dorsally above the hepatopancreas and below the pericardium. Gonads are the hollow enclosing remains of coelom, they are continuous with their ducts because of the reduction of coelom.
Male Reproductive Organs:
The male reproductive organs consist of a pair of testes, a pair of vasa deferentia, and a pair of vesiculae seminales.
(i) Testes:
The two testes are soft, white, elongated structures lying above the posterior half of the dorsal surface of the hepatopancreas and below the pericardial sinus and the heart. Anteriorly they extend as far as the renal sac and posterioly as far as the first abdominal segment.
At their anterior ends, the two testes fuse to form a common lobe, while posteriorly they remain separate. In the middle region, the two testes are separated apart so as to enclose a space through which extends the cardio-pyloric strand.
Histologically, testis reveals that it consists of a large number of lobules compactly held together by the connective tissue. Each lobule has a thin wall consisting of an outer limiting membrane and an inner single layered epithelium enclosing a large central cavity. The lumen of the lobules is always filled with spermatocytes.
Each spermatocyte gives rise to a single spermatozoon. A mature sperm consists of a hemispherical cytoplasmic mass containing a large, dark, crescentic nucleus and a short blunt tail-like process.
(ii) Vasa Deferentia:
From the posterior end of each testis arises a long coiled and narrow tube, the vas deferens. Soon after emerging from the testis, each vas deferens forms a much coiled mass and then runs vertically downwards between the thoracic wall on the outer side and the abdominal flexor muscles on the inner side.
(iii) Vesicula Seminales:
On reaching near the coxa of the last pair of walking legs, each vas deferens swells into a small club-shaped structure, the vesicula serminalis, in which the sperms are stored in the form of white compact bodies called the spermatophores.
Each vesicula seminalis opens to the outside by a male genital aperture on the arthrodial membrane of the coxa of the fifth walking leg of its side. Each male genital aperture is covered by a small flap of integument.
Female Reproductive Organs:
The female reproductive organs consist of a pair of ovaries and a pair of oviducts.
(i) Ovaries:
The two ovaries are white compact and sickle-shaped structures. They are situated above the hepatopancreas and below the pericardial sinus and heart. Both the anterior and posterior ends of the ovaries touching each other and leaving a gap in the middle for the passage of the cardio-pyloric strand.
The shape and size of the ovaries vary considerably according to the age of the prawn and the season of the year. Like the testes, the ovaries extend anteriorly up to the renal sac and posteriorly up to the anterior margin of the first abdominal segment.
Histologically, ovary shows that it is composed of a number of radially arranged rows of ova in various stages of development.
The immature ova lie towards the centre, while the mature ones towards the periphery. Each developing ovum is being surrounded by a number of small nutritive cells. A mature ovum has a rounded appearance with a large nucleus and a cytoplasm full of yolk- granules. Each ovary is enclosed within in a membranous capsule.
(ii) Oviducts:
Both the oviducts are short, wide and thin-walled tubes. Each oviduct arises from the ovary at about the middle of its outer border having a broad funnel at its commencement. It runs vertically downwards and opens by a female genital aperture on the inner side of the coxa of the third walking leg of its side.
12. Fertilisation and Development of Palaemon Malcolmsonii:
The breeding season in Palaemon malcolmsonii is during the months of May, June and July. Fertilisation is external. In copulation a male puts the female down on her back and deposits sperms on the ventral surface near the female genital pores. As the eggs come out of the female genital pores, they are fertilised by the sperms already present there.
The female lays eggs in large numbers. The eggs are large with much yolk forming a central core, the yolk is surrounded by peripheral protoplasm. The fertilised eggs are fastened to the pleopods by a sticky secretion of the tegumental glands. During breeding season a single female prawn is found to carry several hundreds of eggs and developing embryos.
The development is direct and the embryos remain attached to the pleopods until they hatch. A newly hatched embryo looks like a small prawn with all its appendages except the last abdominals, i.e., sixth abdominals. The adult form is reached after a series of moults.
13. Endocrine System of Palaemon Malcolmsonii:
Palaemon Malcolmsonii, like other crustaceans, produces a large number of hormones. It is believed that the sinus gland at the base of the eye-stalk is known to have a number of hormones.
They are supposed to control the spread of pigment in the chromatophores of the epidermis and in the compound eyes. They also seem to have some regulatory power over moulting and affect the deposition of lime salts in the exoskeleton. The blood probably distributes the hormones like the higher forms.
The exact mechanism by which the physiological processes are carried out is still very obscure. Extensive research indicates that there is an X-organ in the eye-stalk along with the sinus gland. Neurosecretory cells in the X-organ and in the brain produce a moult-preventing hormone which is stored in the sinus gland.
When eye-stalks are removed experimentally from a non- moulting specimen, moulting will occur in a few days because the inhibiting effect of the hormone is removed; when eye-stalks from non-moulting specimen are implanted into the body of an eye- stalk-less specimen, moulting is delayed.
A Y-organ, which produces a moult-accelerating hormone, has been described in some crustaceans. The interaction of the moult-preventing and moult-accelerating hormones may be the regulatory device in the moulting process.
For the expansion and contraction of the pigments into and out of the chromatophore processes, certain chromatophorotropic hormones in the sinus gland appear to be responsible as revealed by the removal of the eye-stalks (darkening effect) or by the injection of eye-stalk extracts (paling effect). A specific hormone from the sinus gland is known to control the retinal pigment movements also.
