Asterias: An Example of Phylum Echinodermata

In this article we will discuss about Asterias:- 1. Habit and Habitat of Asterias 2. External Structures of Asterias 3. Structure of Body Wall 4. Coelom 5. Digestive System 6. Respiratory System 7. Locomotion 8. Circulatory System 9. Excretory System 10. Nervous System 11. Reproductive System 12. Development.

Contents:

1. Habit and Habitat of Asterias
2. External Structures of Asterias
3. Structure of Body Wall of Asterias
4. Coelom of Asterias
5. Digestive System of Asterias
6. Respiratory System of Asterias
7. Locomotion of Asterias
8. Circulatory System of Asterias
9. Excretory System of Asterias
10. Nervous System of Asterias
11. Reproductive System of Asterias
12. Development of Asterias

1. Habit and Habitat of Asterias:

Asterias is an exclusively marine and widely distributed member of echinoderm. All the species under this genus are benthonic animals, because they inhabit the bottom of the sea. They are quite abundant on various types of sea-bottoms, specially at places where bivalves are available as food. They are carnivorous and predacious animals.

They prefer to stay on rocky or stony places where they can hide very easily and lead a sluggish life. Majority of the forms are photonegative and prefer to live in shaded areas. A few exceptions are the Asterias rubens, Asterias gibbosa, Asterias panceri, where positive re­sponse to light is observed.

2. External Structures of Asterias:

Asterias has a five-pointed star-shaped body (Fig. 21.2A). The body consists of a central disc and five symmetrically placed arms (or rays). Sometimes individuals with more than five arms are found in nature. Many starfishes have even four arms. Leptasterias has a six-rayed body.

The genus Solaster has seven to fourteen-rayed body and Pycnopodia helianthoides possesses fifteen to twenty-four arms. Occurrence of less than five arms may be possibly due to mechanical injury. The arms are conspicuously broad at their bases and they gradually taper towards the tips.

The body has two surfaces, the upper convex and much darker side is called aboral or abactinal side. The other side is flat; less pigmented and is designated as oral or actinal side. The body exhibits radial symmetry. The imaginary lines dividing the central disc and the arms are called the radii and the intervening regions between the radii are called inter-radii.

The body is cov­ered by a hard and tough covering contain­ing numerous calcareous plates or ossicles derived from mesodermal tissue. At the cen­tre of the oral surface of the central disc, a five-rayed aperture called mouth or actinosome (Fig. 21.2B) is present. This open­ing is surrounded by a membranous peristome and it also possesses five groups of oral papillae.

Five narrow grooves, called ambulacral grooves, one on each arm, run orally from the five-rayed aperture to the extremity of the arms. The edges of these grooves are provided with two or three rows of movable calcareous ambulacral spines. Be­sides the ambulacral spines, rows of immov­able stout spines are also present.

The aboral side contains numerous irregular rows of short and stout spines supported by ossicles. The oral and aboral surfaces of the body are separated by a row of prominent spines.

Many dermal pores are situated in the spaces between the ossicles on the aboral side. From each dermal pore projects a very small deli­cate, finger-like, membranous retractile proc­ess, called dermal branchia or papula. It is respiratory in function.

A minute aperture, called anus, is situ­ated at the centre of the aboral surface. On the same side of the body, between the bases of two of the five arms (bivium), there lies a structure, called madreporite (Fig, 20.2A) and the rest of three arms are known as trivium.

The madreporite is a flat disc-like body with radiating grooves. The presence of more than one madreporite in some spe­cies is possibly due to increase of the number of arms beyond the normal number of five.

Scattered all over the body surface there are many microscopic, pincer-like bodies, called pedicellariae (Fig. 21.3A), each consists of a flexible stalk and three calcareous pieces —one basilar and two jaws or valves. The jaws are attached with the basilar piece. The pedicellariae are modified spines.

This par­ticular variety of pedicellariae is called the pedunculate type. Two types of pedunculate pedicellariae are encountered in Asterias (Fig. 21.3B).

These are:

(1) Straight Pedicellariae— when the two jaws remain more or less straight on the basilar piece.

(2) Crossed Pedicellariae—when the basal portions of the jaws are curved and cross each other.

The pedicellariae are protective organs. They help to clean the surface of the body of debris, sand grains, etc. which would inter­fere respiration and other activities. They also help in the capture of small prey. The jaws are movable on the basilar piece. The jaws are operated by two sets of muscles— two pairs of adductor muscles and one pair of abductor muscles.

In natural condition and in well-preserved state two double rows of tube-feet or podia are seen in the ambulacral grooves. Each tube-foot or po­dium has a soft tubular body and it ends in a sucker. These are locomotor organs and are capable of great extension. At the tip of the ambulacral groove, sense organs are placed.

A minute red spot, called the eye, is present at the extremity of each ambulacral groove on the oral side. The eye is composed of several ocelli. Just above the eye lies a median unpaired non-retractile, hollow projec­tion, called terminal tentacle. This tentacle resembles a tube-foot but lacks the sucker and is regarded as tactile and olfactory organ.

3. Structure of Body Wall of Asterias:

The body is covered externally by cuticle. The cuticle is differentiated into an outer thick homogeneous layer and a very delicate inner layer. The epidermis is situated be­neath the cuticle. The shape and structure of the cells composing of the epidermis vary greatly at different regions of the body.

Usu­ally these cells are of ciliated columnar type. Besides them, several other cells are also present. The cells are neurosensory cells, pigment cells and glandular cells. The gland cells are of two varieties—goblet or mucous cells and muriform gland cells. The muriform types contain coarse granules and the goblet types have very fine granules. Beneath the epidermis lies a network of nerve fibrils.

The dermis is comparatively thicker. It is meso­dermal in origin and is composed of fibrillar connective tissue. The epidermis and the dermis are separated by a delicate basement membrane.

The dermis contains the endoskeletal ossicles and also possesses a system of canalicular haemal spaces. Beneath the dermis lies the muscular layer. This layer is divided into an outer circular muscle layer and an inner longitudinal muscle layer. The innermost layer of the body wall is lined by the coelomic epithelium.

Skeleton of Asterias:

The rigidity of the body of Asterias is due to the presence of definite skeleton. This supporting skeleton comprises of the main deeper skeletal elements embedded in the dermis and the superficial skeleton in the forms of spines, warts and tubercles which are borne on the deeper skeleton. The ossicles are of various shapes and bound together by connective tissue.

The ossicles have distinct pattern of ar­rangement. The mouth is surrounded by five plate-like oral ossicles. Each ambulacral groove is supported by double rows of rod-­like ambulacral ossicles. These ossicles are articulated with the adambulacral ossicles at their aboral ends like an inverted ‘A’. The apex of the ‘A’ projects into the coelom and forms a prominent ambulacral ridge.

The ambulacral ossicles are movably articulated so as to allow the closure and opening of the ambulacral groove. A row of lateral adambulacral ossicles meets the ambulacral ossicles on their outer ends. The adambulacral ossicles bear movable spines. In some forms of Asterias, one or two rows of additional ossicles are present. They are called supra- marginal and infra-marginal ossicles depend­ing on their position.

The marginal ossicles form the dorso-lateral sides of the arms and usually remain covered by small ossicles. A series of carinal ossicles forms the mid- dorsal skeleton of the arms. Between the carinals and the marginal ossicles there may be a number of elongated dorso-lateral ossicles.

The arrangement of skeletal elements on the aboral side is indistinct in adult, but in a freshly metamorphosed Asterias the ar­rangement is distinct. At the middle of this side a central plate is present which bears the anus. This central plate is surrounded by five interradial plates. One of such plates incorpo­rates the madreporite. The interradial plates are surrounded by radially placed terminal plates.

In starfishes where the number of arms exceeds five, the interradial and termi­nal plates increase with the number of arms. As the arms grow in length the terminal plates are also shifted to the tip and addi­tional row of plates develops behind the terminals. These plates which develop be­hind the terminals constitute the carinals of the mid-dorsal line of the arms.

Section of Arm of Asterias:

If one of the arms of Asterias is sectioned the following structures become visible (Figs. 21.4 and 21.5):

1. The wall of the arm shows the appear­ance of an arch with its convexity upwards. The aboral side is thicker than the oral side.

2. The body wall consists of epidermis, dermis, muscular layers and coelomic epi­thelium. The epidermis consists of ciliated columnar cells, neurosensory cells, pigment cells and two types of gland cells. Above the epidermis there is the cuticle. Beneath the epidermis lies the basement membrane. The dermis is mesodermal in origin and is fibril­lar in nature.

The dermis is thicker than epidermis and contains haemal spaces and endoskeleton in the form of ossicles. The muscular layer is situated beneath the der­mis. The muscular layer is differentiated into an outer circular layer and an inner longitu­dinal layer. The coelomic epithelium forms the inner lining of the body wall.

3. The surface of the body bears spines, tubercles, pedicellariae and warts which are borne on the deeper skeletal elements.

4. The deeper skeleton consists of ossicles of various shapes. The ambulacral groove is supported by two rod-like ambulacral ossicles, articulated at their aboral ends like an inverted ‘A’ with the upper transverse ambulacral muscle.

Two adambulacral ossicles, one on the lateral side of each ambulacral ossicle, are present. The adambulacral ossicles bear movable spines. The upper dorso-median side of the arm contains carinal ossicle. Between the carinal and adambulacral ossicles there exist supra and infra-marginal ossicles which form the lateral walls of the arm.

5. Projecting between the dermal ossicles there are many hollow outgrowths of the body, called respiratory papulae. They are quite abundant on the aboral surface.

6. The spacious cavity enclosed by the body wall is called coelom. Scattered coelomocytes are present in the coelomic space.

7. In the ambulacral groove there are double rows of tube-feet. The tube-foot has a tubular body ending in a sucker. Each tube- foot is continuous with a bladder-like am­pulla situated in the cavity of the arm.

8. In the cavity of the arm, there are two pyloric caeca which remain suspended from the aboral body wall by two mesenteries.

9. Two gonads are located in the cavity of the arm between the pyloric caeca and the ampullae.

10. Towards the oral side, sectional views of radial canal and lateral or podial canals of the water vascular system are seen.

11. Towards the oral side, sectional view of the radial nerve, Lange’s nerve, hyponeural sinus with its dividing septum containing radial haemal strand are present.

4. Coelom of Asterias:

Asterias has a spacious coelom. The coe­lom is enterocoelic in origin. It surrounds the alimentary canal and the gonads. Besides this main perivisceral coelom, there are many small coelomic cavities like water vascular system, axial sinus, perihaemal cavities and genital sinuses.

The inner surface of the perivisceral coelom is lined by ciliated epi­thelium which is differentiated into two sepa­rate layers. The parietal layer covers the inner surface of the body wall and the vis­ceral layer forms an investment for the dif­ferent organs contained in the body. The coelom is filled up with coelomic fluid. This fluid is chemically similar to sea water in composition and contains amoeboid coelomocytes.

However, potassium is present in higher concentration than in sea water. Traces of amino-nitrogen are reported to be present in asteroids.

A small amount of reduc­ing sugar is present in perivisceral fluid of Asterias forbesi. Excretory products like urea and ammonia are also present in coelomic fluid. The coelomocytes are highly phagocytic and are supposed to be excretory in function.

5. Digestive System of Asterias:

The digestive system (Fig. 21.6A) includes alimentary canal and digestive glands. The alimentary canal is a very short straight tube. It extends from the oral to the aboral side of the body. The mouth is located near the centre of the oral surface. The mouth leads into a very short and wide oesophagus. The oesophagus passes into a spacious stomach.

The stomach is distinctly divided into two parts by a horizontal constriction. The volu­minous anterior portion is called cardiac stomach and the smaller posterior chamber is called pyloric stomach. The cardiac stom­ach is a five-lobed sac. Its inner wall becomes greatly folded. The cardiac part is attached to the ambulacral ridge by two gastric ligaments and is capable of being everted through mouth.

The cardiac stomach com­municates with the pyloric stomach. The pyloric stomach is a pentagonal sac and each angle is produced into a pair of large glan­dular appendages. There are, in all, ten such glandular structures. They possess various names such as pyloric caeca or digestive glands or brachial caeca or hepatic caeca (Fig. 21.6B). The pyloric caeca are glandular in nature and secrete digestive enzymes.

They start as cylindrical duct which imme­diately divides into two tubular stems. The tubular stems give off two series of short lateral branches. The stem extends up to the tip of the arms. The lateral branches dilate at their tips to form numerous small pouches.

The pyloric caeca are attached’ with the abo­ral wall of the arm by mesenteries (Fig. 21.6C). The pyloric caeca have the typical histological picture of the alimentary canal, but the lining epithelium is very thick. It is lined by four types of cells.

The cells are:

(1) Flagellated cells—with long flagella to maintain a constant circulation of fluid.

(2) Secretory cells—produce digestive en­zymes.

(3) Mucous cells—secrete mucus.

(4) Storage cells—contain lipid, glycogen, and polysaccharide-protein complex as stored food.

Heyde (1922) performed a series of experiments on digestion in Asterias and advanced the idea that proteolylic activity is encountered in both stomach and pyloric caeca. But the other enzymes, like amylase, lipase, etc. are not found.

The pyloric stomach opens into a short conical intestine which opens to the exterior through anal aperture situated on the aboral side. From the intestine, two hollow intesti­nal caeca are given off inter-radially. These intestinal caeca are provided with several short irregular branches.

The greatly folded inner lining of the intestinal caeca contains mucous and gland cells. These are brownish in colour and secrete brown coloured fluid. The exact physiological role of these struc­tures is not known but, with all probability, they help in excretion. These structures are homologised by many, with the respiratory trees of the holothurians.

Histology:

The histological picture of the alimentary canal reveals that the outer layer is com­posed of the visceral layer of the coelomic epithelium. The next layer is the muscular layer and the innermost layer comprises of endoderm or enteric epithelium.

6. Respiratory System of Asterias:

Respiration is carried on by numerous papulae (or called gills or dermal branchiae). The papulae are delicate hollow outgrowths of the body wall (see Fig. 21.3C) and project between dermal ossicles. These are distrib­uted abundantly on the aboral side of the body. The histological structure is same as that of the body wall excepting the thinness of the dermis and the absence of dermal skeleton.

The lumen of each papula is con­tinuous with coelom and is lined by ciliated coelomic epithelium. Through these respira­tory papulae exchange of oxygen and car­bon dioxide takes place by diffusion. In addition to the papulae, the water vascular system also helps in respiration. Meyer (1935) proved that the tube-feet of each ambulacral groove in Asterias rubens are responsible for approximately 10% of total O₂ uptake.

7. Locomotion of Asterias:

The mechanism of podial locomotion in asteroids has been described by J. E. Smith (1947). The same mechanism operates in other groups. The water vascular system in Asterias sp. operates on the principle of hy­draulic pressure during movement. This system is indirectly concerned with the loco­motion of the animal.

The water vascular system is always filled with fluid and the loss of fluid across the wall of tube-feet during fluid pressure elevation is instanta­neously compensated by the intake of water into the system through the madreporite.

In Asterias sp., locomotion is caused by the action of tube-feet. These are present as two double rows on each ambulacral groove. The tube-feet are capable of being extended out­wards and forwards to the direction in which the animal moves.

Locomotion on hard horizontal surface:

By alternate extension and contraction of all the tube-feet, the starfish progresses quite steadily over the surface. When the animal moves to a particular direction, the arms on that side are lifted from the surface and the tube-feet become greatly extended.

The ex­tension of the tube-feet is caused by the contraction of ampullary circular muscle fibres of ampullae. Due to contraction, the fluid from the ampullae is forced into the podia.

The presence of valves at the bases of the lateral vessels prevents re-entry of water into the podial canals. As the tube-feet are extended by hydraulic pressure and extend towards the direction of movement, the suck­ers at the ends of tube-feet adhere to the substratum. The suckers produce vacuum cup at the centre.

The longitudinal or retrac­tor muscle fibres of tube-feet alternately retract and the tube-feet are used to take a firm grip largely through ionic interactions and suction on the hard substratum and push the body forward. The tube-feet finally relax and release their holds on the substra­tum.

The tube-feet again extend forward to apply the suckers to the new places by the same fashion. By such alternate process of coordinated contraction and relaxation of podia, the animal moves very slowly (Fig. 21.9). This type of progression is possible when the animals move on hard surface under water.

During progression the tube- feet work in this manner by extending to­wards one direction. This co-ordinated ac­tion of the podia and ampulla is controlled by the nerve ring and the radial nerves (Fig. 21.7C). It is also observed that the podia are lined by long gland cells which secrete sticky fluid to lubricate the process.

Adhesion and detachment of the podial suckers from the substratum are controlled by two chemical substances which are se­creted from the tube-feet (podia). One chemi­cal substance provides the adhesion to the substratum and the release of another sub­stance helps to break the adhesion and brings about the detachment.

In Asterias sp., the tube-feet (podia) are provided with suckers. In other mud-dwelling forms the suckers become inactive and utilise the tube-feet as miniature legs.

Jennings (1907) and Kerkut (1953) have stated that the tube-feet do not pull the sea star along but their action is rather that of stepping, bringing the animals forward by a backward push.

Locomotion on vertical surface:

Asterias sp. can climb rocks vertically by protraction and retraction of circular, retrac­tor and postural muscles of the ampullae and podia respectively. They employ a pow­erful force on podial suckers for adhering tenaciously to rocks. Due to the suction-like mechanism of the podial suckers the animal can climb rocks vertically (Fig. 21.10).

Food gathering:

In the echinoderms with open ambulacral grooves, the podia primarily have a food gathering function. They feed on microscopic organisms and fine particles by mucociliary method. Mucus prospects the surface to collect the small particles. Giston (1924) recorded it in Asteroids and Crinoids.

8. Circulatory System of Asterias:

A true blood-vascular system is lacking in echinoderms. Tine system which is respon­sible for the transportation of the digested food to the different parts of the body is called circulatory system. The circulatory system of echinoderms is represented by a perihaemal system (Fig. 21.11 A) and a hae­mal system (Fig. 21.11 A).

Perihaemal System:

It is peculiar only to the echinoderm group where it is well-developed. The perihaemal system is derived from the coelom and consists of a series of channels which are lined with the ciliated epithelium and enclose the water vascular system and haemal system except the gastric haemal tufts.

These tubes of coelom are generally called “perihaemal system”. But it is inter­esting to note that neither of these systems comprises vessels with contractile walls and there is no definite circulation of the con­tained fluid.

General plan:

When typically developed, perihaemal or pseudohaemal system (as in Asteriodea and Ophiuroidea) consists of:

1. A ring-like circumoral canal (sinus) lying around the mouth.

2. Five radial perihaemal canals arise from the outer oral ring sinus along the radii between the radial nerve cords and the ra­dial vascular trunk, together with:

(i) An axial sinus communicating with the exterior by the water pore or pores.

(ii) Aboral ring vessels (= sinuses). These vessels are situated at the aboral side and of the nature of specialized part of the coelom, from which they are de­veloped and have definite epithelial lining.

Origin of perihaemal system:

Development of perihaemal system is best studied in Asteriodea.

(i) In larvae, the left posterior coelom gives off four inter-radial prolon­gations, each of which bifurcates to produce to the adjacent arm rudiments. These are the first traces of the outer perihaemal canals.

(ii) The aboral ring sinus is an outgrowth of the left posterior coelom from which it becomes completely cut off in the adult.

(iii) The anterior coelom, with the atrophy of preoral lobe, becomes much re­duced in size and persists as the axial sinus.

(iv) The inner perihaemal ring is an out­growth of the axial sinus, with the ventral end of which it remains con­tinuous.

Main sinuses of perihaemal system:

The main sinuses of the perihaemal sys­tem are:

(i) Axial sinus:

It is a thin-walled, wide tubular coelomic cavity passing almost ver­tically from the subactinal (suboral) to the actinal (oral) surface in the interradius of the madreporite. It is lined by the ciliated epithe­lium. The stone canal and axial organ (= axial gland) project into it, being attached to its inner wall.

The axial sinus along with the axial organ and stone canal constitute the axial complex. Orally the axial sinus is con­tinuous with the inner perihaemal space which surrounds the mouth. At its aboral end it communicates with the stone canal on the one hand and with the exterior through some of the pores of the madreporite on the other.

(ii) Aboral perihaemal ring sinus:

It is a thin-walled, pentagonal or circular channel (sinus) placed at the aboral side between stomach and the skin and encloses the aboral haemal ring sinus. It is associated with geni­tal sinus and communicates with the axial sinus.

(iii) Genital sinus:

The aboral ring sinus gives off 5 pairs of small, slender genital branches that surround the gonads and the genital haemal strands. Each pair develops from each arm.

(iv) Oral perihaemal ring sinus:

It is a ring-like, large tubular sinus running around the mouth. It encloses the oral haemal ring sinus. The oral end of the axial sinus opens into the inner part of the oral ring sinus.

The oral perihaemal ring sinus is divided into two by a slightly oblique septum. The inner perihaemal ring is narrow and an outer perihaemal ring is large and wide. The outer perihaemal ring is made up of as many inter- radial pieces as there are arms, each piece being prolonged into two adjacent arms.

(v) Radial perihaemal sinuses:

Each arm has a prolongation of two pieces of the outer perihaemal ring and five radial perihaemal sinuses develop from the outer perihaemal ring. Each radial perihaemal sinus extends upto the tip of each arm and is divided into two by the radial haemal strand. It is lined by flat epithelium, whether ciliated or not is not known.

(vi) Lateral channels:

These channels arise from the radial perihaemal sinuses and sup­ply the tube feet.

(vii) Marginal sinuses:

In each arm, there are two longitudinal, marginal sinuses on each side lying below the marginal nerve cord. These sinuses open into the radial perihaemal sinuses of their respective arms.

(viii) Peribranchial sinuses:

These sinuses are situated around the basal parts of the papulae.

Significance of Perihaemal System:

Previously it was considered that perihaemal system constitutes a sort of ac­cessory circulatory system. But Lang (1896) has stated that both the perihaemal canals and epineural canals are in essential service in protecting the nerve trunks from being pressed or torn.

Hyman (1955) has also re­ported that it is entirely erroneous, except in the sense that all coelomic spaces contribute something to circulation, respiration and excretion. She is of opinion that the term ‘perihaemal’ is confusing and should be re­placed by the term “hyponeural canals” because of their relation to the nervous sys­tem and to avoid un-wasted implications.

According to her the hyponeural system of canals appears functionally related to the nervous system, “probably acting to cushion against injury”. But she further remarks that “the evidence of the haemal channels inside the coelomic channels remains mysterious.”

Haemal System or Blood Lacunar System:

The blood lacunar system as such is absent in Asterias, but its function has been taken up by a special system, called haemal system. This system of channels is enclosed by coelomic spaces (Fig. 21.11A). These spaces are sometimes designated as perihaemal system. The haemal channels are lined by coelomic epithelium with an inner connec­tive tissue layer (Fig. 21.11C).

The haemal channels are actually inter-communicating spaces and are not true blood vessels. They develop like the haemocoel of other animals. So the term perihaemal system comprising the cavities enclosing the haemal channels seems rather confusing.

The main haemal sinuses are as follows:

(i) Oral haemal ring:

It is in the form of a circular vessel around the mouth which proceeds in the septum dividing the hyponeural ring sinus. The oral haemal ring gives off radial haemal sinuses into the arms. Each arm has one radial haemal sinus which is situated in the septum of the hyponeural radial haemal sinus and gives off branches to the tube feet or podia.

(ii) Aboral haemal ring:

In the aboral side there is an aboral haemal ring (Fig. 21.11A) which is pentagonal and runs inside the aboral perihaemal ring sinus, situated be­neath the aboral surface of the central disc. It gives off five pairs haemal branches (geni­tal haemal strands) to the gonads. The geni­tal haemal strand plays a role in gamete nutrition. The haemal fluid helps in the stor­ing of nutrients and in the distribution of digested food.

The oral haemal ring is communicated with the aboral haemal ring by an ascending haemal plexus in the axial gland. Two gas­tric haemal tufts open into the haemal plexus of the axial gland towards its opening into the aboral haemal ring. The gastric tufts are the only part of the haemal system which is not enclosed by the so-called perihaemal cavities.

According to some authors, about twenty pyloric haemal channels are in com­munication with the axial gland and haemal plexus through gastric haemal tufts. Each pyloric caecum is attached with the aboral wall of the arms by a pair of mesenteries and each mesentery contains a pyloric haemal channel at the base. Through these channels digested food is conveyed to the haemal system.

The cavities of the haemal channels are filled with coelomic fluid containing coelomocytes (Fig. 21.11C). This fluid in the haemal system undergoes slight movement and helps in the distribution of digested foods to different parts of the body.

The haemal plexus of the axial gland is regarded as the ‘heart’ of the haemal system. Besides the axial gland, contractility of the terminal process of the axial gland, gastric haemal tufts, aboral haemal ring are also observed by some workers.

Axial Complex:

Asterias has a very well-formed axial com­plex comprising a coelomic cavity enclosing the stone canal and axial gland (Fig. 21.11C). The axial complex remains intimately asso­ciated with the inter-brachial septum. The axial sinus is a thin-walled tubular cavity which opens orally into the inner smaller ring of the hypo-neural ring sinus. It opens aborally into the genital sinus and finally into the ampulla of the stone canal.

The axial gland (or axial organ) is a long spongy body of brown or purple colour and gives a small terminal process at the aboral end. This proc­ess is surrounded by a contractile closed sac named as terminal sac or dorsal sac or madreporic vesicle. This vesicle is situated very close to the ampulla of the stone canal, but has no connection with it. The axial gland terminates orally in the septum of the hypo-neural ring sinus.

The axial gland (or axial organ) is variously called as heart, ovoid gland, brown gland, septal organ and dorsal organ. The function of the axial gland and its relation to other systems are still disputed.

It has an interior core of connective tissue traversed by many spaces containing coelomocytes and externally it is covered by a coelomic epithelium. The histological pic­ture reveals its similarity with the haemal system and both are closely associated.

9. Excretory System of Asterias:

Definite excretory organ is lacking in Asterias. By injecting various coloured sub­stances into the coelomic fluid, it is observed that the coelomocytes play the main role in excretion. The intestinal caeca, in addition to other functions, extract waste products from the coelomic fluid and eliminate the wastes through alimentary canal.

The nitrogenous waste products diffuse out from the body into the coelomic fluid. These are immediately engulfed by the wan­dering coelomocytes and pass out of the body through the papulae. The nitrogenous wastes include ammonia, urea and traces of uric acid. The presence of creatine and crea­tinine in Asterias is very significant, because these substances are not found in any other invertebrates.

10. Nervous System of Asterias:

The nervous system of Asterias is very simple. It includes only nerve net and has no central ganglionic formation. The nervous system consists of three systems oral or ectoneural system, deep or hypo-neural system and coelomic nervous system.

(i) Oral or Ectoneural Nervous System:

This system includes the main part of the nervous system and is situated below the epidermis. This includes a nerve pentagon situated in the peristomial membrane surrounding the mouth. The nerve pentagon at each radius gives off a radial nerve which runs as a slender thick band throughout the ambulacral groove.

The radial nerve is situated just below the radial canal as median integumen­tary thickening. The radial nerve ends as a sensory pad or cushion at the base of the terminal tentacles. The radial nerve gives branches to the tube-feet and becomes con­tinuous with the sub-epidermal nerve plexus of the body wall.

The nerve pentagon and the radial nerves consist of nerve fibrils with scattered bipolar and multipolar nerve cells. The radial nerve appears like a thick ‘V’- shaped body below the hypo-neural sinus. It is separated from the hypo-neural sinus by a thin dermis and coelomic epithelium.

(ii) Deep or Hypo-neural Nervous System:

The different nerves composing this system are motor nerves. This nervous system com­prises of Lange’s nerves. Lange’s nerve is a plate of nervous tissue situated beneath the coelomic epithelium of the hypo-neural sinus and forms a lateral lining on the wall of this sinus. Lange’s nerve extends as five inter- radial nerve thickenings above the main nerve pentagon of the ectoneural system. Each Lange’s nerve innervates the muscles of the corresponding arm.

(iii) Coelomic Nervous System:

The sub-epi­thelial nerve plexus at the outer ends of the ambulacral ossicles forms marginal nerve cord on two sides of the arms. The marginal nerve cord gives off lateral motor nerves which proceed to the aboral side between the ad ambulacral and ambulacral ossicles to reach the coelomic lining. Beneath the entire coelomic lining the nerves form plexus which innervates the body wall musculature and the gonads.

Sense Organs:

The neurosensory cells, terminal tenta­cles and the eyes are the sensory units in Asterias.

Neurosensory cells:

These cells are tac­tile in function and act as chemoreceptors. They are present throughout the body, but are quite abundant in the suckers of the tube- feet in the ambulacral regions, in the epider­mis, at the base of pedicellariae and spines. Each neurosensory cell has a slender fusi­form body. The proximal end is connected with sub-epithelial nerve plexus and the distal end goes up to the cuticle.

Terminal tentacles:

They are regarded as tactile receptors. These sensory organs help to survey the environment during locomotion.

Eyes:

They constitute the major sense organs. At the base of each terminal tentacle towards the oral side a pigmented eye spot is present. Each ‘eye’ consists of numerous photoreceptors in the forms of cup-like pigmented ocelli. Each ocellus is covered over by cuticle.

The wall of the cup is com­posed of red pigmented cells and retinal cells. The epidermis beneath the cuticle of the cup may form lens which may be absent in some cases. The number of ocelli per eye may be about 80-200. The number may even increase with age.

11. Reproductive System of Asterias:

Reproduction, in Asterias, is mainly sexual but asexual reproduction by splitting of the body also takes place. The sexes are separate (gonochoristic) but sexual dimorphism is absent.

The ovaries and testes are similar in appearance. They are situated in the same place of the body. There are ten testes or ovaries, two in each arm of the body. The proximal ends of the gonads are attached to the aboral body wall near the interbrachial septum.

Each gonad is an elongated branched body which becomes considerably enlarged at maturation stage. The gonad is enclosed in a coelomic genital sac. Microscopical exami­nation reveals that the gonad is lined by a germinal epithelium with a connective tissue matrix containing germ cells.

Very near to the point of attachment with the interbrachial septum, each gonad gives out a very short ciliated gonoduct which opens to the exte­rior through gonopore situated on the aboral side at the angle of the arms.

Fertilization:

The sex cells are set free into the sea water and the fertilization is external.

12. Development of Asterias:

The developmental sequences are best known in an allied form of Asterias, Asterina gibbosa (Fig. 21.12). The eggs are spherical and yolky. Each egg measures about half of a millimetre in diameter. After fertilization the zygote divides into two. The cleavage is total and is practically equal. The two cells produced by first cleavage, divide to form four cells.

The process of such division (ra­dial) goes on until thirty-two cells are pro­duced. These cells are arranged in such a manner that they enclose a central cavity (blastocoel) and this stage is called blastula. The outer surface develops vibratile cilia and the inner cavity increases by repeated cell divisions.

Gastrulation starts as a process of invagination and a double-walled gastrula is formed. The invaginated sac forms the archenteron and opens to the exterior through the blastopore. The gastrula elongates and the cilia over the surface of the body become restricted along definite bands. The gastrula is transformed into a larva in course of time.

The shape of the larva gradually changes. With the elongation of body, the blastopore, at the middle of the two poles, becomes subsequently shifted to the posterior end. The outer layer is converted into ectoderm. The space between the ectoderm and the endoderm becomes occupied by the mesen­chyme.

The cavity of the gastrula becomes distinguishable into two parts, the arch­enteron and the enterocoel the narrower proximal part is called the archenteron an the wider terminal part is called the enterocoel. The blastopore becomes the larval anus. The enterocoel gives off right and left lateral pouches, called the enterocoelic pouches.

The enterocoel becomes completely closed off from the gut and becomes divided into three parts, an anterior part and two lateral (right and left) parts in the form of pouches. The left pouch grows faster than the right and proceeds posteriorly in the space be­tween the body wall and the alimentary canal.

The pouches coalesce posteriorly to give origin to the coelom in adults. The anterior undivided portion becomes subse­quently cut off from the lateral pouches and forms the coelom of the pre-oral lobe. This part gives off a five-lobed extension, called hydrocoel, on the left.

From the hydrocoel the entire adult ambulacral system devel­ops. Two apertures appear on the surface— one is the mouth on the ventral side and the other on the dorsal side, called the dorsal pore. The mouth opens into the larval stom­ach and the larval anus soon closes.

The particular type of larva in starfishes is called Bipinnaria larva. It has bilaterally symmetri­cal lobes with ciliated bands. The Bipinnaria is a feeding larva which is succeeded by Brachiolaria stage. The detailed structures of these larval forms have been discussed separately in the general notes Echinoderms.

The pre-oral lobe of the larva assumes an elongated cylindrical larval organ. An eleva­tion, the rudiment of sucker, develops at the middle of the larval organ. The sucker helps in the attachment of the larva during meta­morphosis.

The larva is transformed into the five-rayed starfish by the elimination of the pre-oral lobe, by the development of the arms and tube-feet and by changing the internal structures. The larval mouth disap­pears and a new mouth is formed at the centre of the hydrocoel. The permanent anus is formed on the dorsal side.