Echinus: An Example of Phylum Echinodermata

In this article we will discuss about Echinus:- 1. Habit and Habitat of Echinus 2. External Structures of Echinus 3. Body Wall 4. Coelom 5. Digestive System 6. Respiratory System 7. Water Vascular System 8. Haemal System or Blood Lacunar System  9. Locomotion 10. Excretory System 11. Nervous System 12. Reproductive System.

Contents:

1. Habit and Habitat of Echinus
2. External Structures of Echinus
3. Body Wall of Echinus
4. Coelom of Echinus
5. Digestive System of Echinus
6. Respiratory System of Echinus
7. Water Vascular System of Echinus
8. Haemal System or Blood Lacunar System of Echinus
9. Locomotion of Echinus
10. Excretory System of Echinus
11. Nervous System of Echinus
12. Reproductive System of Echinus

1. Habit and Habitat of Echinus:

Echinus is a marine and benthonic ani­mal. They usually live in intertidal zones and may also extend to the depth of about 5000 m. They habitually live in hard or rocky bottoms. They are gregarious forms. They eat all sorts of food available at the bottom of sea. Many small slow-moving and seden­tary animals become the victims of Echinus.

2. External Structures of Echinus:

Echinus has a spherical body with a flat­tened oral surface. The internal structures of the body are housed in a shell or corona. The corona is over-lined by delicate and ciliated epidermis. The corona is composed of plates which are firmly sutured together excepting the two leathery areas, one at the oral side, called peristome and the other on the aboral side, called periproct (Fig. 21.13A).

The mouth is placed at the centre of the peristome (Fig. 21.13B) and the anus is eccentrically placed in the periproct. The peristome bears five pairs of modified tube-feet, called buc­cal podia. They have short bodies with cir­cular disc at the tips and act as the chemo- receptors. Five pairs of small gills project from the peristomial membrane that encir­cles the mouth.

The corona is composed of twenty me­ridional rows of polygonal plates. Five per­forated areas for two rows of tube-feet are recognised. They are separated by wider areas where such perforations for the tube- feet are absent. In living condition five dou­ble rows of tube-feet are distributed in a definite pentaradiate plan from the peristome to the periproct.

The areas where the tube- feet are present are called ambulacral areas or radii. These areas are equally spaced by wider intervening areas between the radii. These areas are called inter-ambulacral areas or inter-radii. The tube-feet or podia are slender and highly extensible bodies which may project beyond the spines in fully stretched condition.

The corona, as stated earlier, is composed of twenty meridional rows of plates. Each radius at the aboral end terminates in one ocular plate, a part of the apical skeleton. In the peristome five pairs of small buccal plates are present. Two such plates are present in each radius. The periproct also contains a few small irregular plates.

Surrounding the periproct there are ten apical plates. Of these plates, five are smaller and are called ocular plates while five are larger and are called genital plates. On these plates there are tubercles bearing spines of various shape and size.

Most of the spines are long and cylindrical. The spines are thickly placed all over the body excepting the peristome and periproct. The spines are solid structures and are pointed at the tip. They are movably articulated and have individual muscles to be operated upon.

Pedicellariae:

Besides the spines, there are long-stalked three-jawed pedicellariae. Several types of pedicellariae are encoun­tered in this group (Fig. 21.13C).

1. Gemmiform type:

Gemmiform type, when the stalk is very stiff and the head is round. Each jaw is provided with poison gland.

2. Tridactyle type:

Tridactyle type, when the stalk is very flexible and the jaws are long.

3. Ophiocephalous type:

Ophiocephalous type, when the stalk is flexible and the jaws are toothed and broad.

4. Trifoliate type:

Trifoliate type, when the stalk is very flexible and the jaws are blunt and toothed.

Sphaeridia:

The ambulacral areas con­tain many small, roundish and transparent solid bodies, called sphaeridia. These bodies are regarded as the sense organs which are possibly responsible for maintenance of bal­ance.

Lantern of Aristotle (Aristotle’s lantern):

The complicated masticatory apparatus of Echinus is known as lantern of Aristotle or called Aristotle’s lantern (Fig. 21.14). It is associated with the mouth and is composed of five strong and sharp teeth whose lower parts are surrounded and supported by the five-sided structures characteristic of the lan­tern of Aristotle. This structure resembles superficially the ancient Greek lantern. Hence the name is lantern of Aristotle.

The lantern comprises of five jaws and five radial pieces, called rotulae, which are joined with the jaws aborally. Each jaw is provided with a tooth. The upward and inward movements of the teeth are caused by the action of the muscles connecting the jaws and the radial arches. The lantern contains coelom as the enlargement of the perihaemal ring.

3. Body Wall of Echinus:

The body wall consists of an epidermis on the outer side, a middle layer of dermis and an inner lining of coelomic epithelium.

The epidermis is single-layered and is formed of cuboidal or columnar cells. These cells are ciliated throughout excepting the terminal discs of the tube-feet and other exposed places where the cuticle is present. The der­mis is composed of connective tissue with scattered stellate cells and contains endoskeleton.

The cells of coelomic epithelium are flattened and flagellated types. Pigment cells (chromatophores) are present in or beneath the epidermis. Regular muscular layer is absent and the muscles are limited to the bases of the spines, pedicellariae and the masticatory apparatus.

4. Coelom of Echinus:

The interior of the body is occupied by a spacious coelom (Fig. 21.15). Besides this major cavity, there are some minor coelomic compartments, viz., peripharyngeal cavity, periproctal sinus, perianal sinus and aboral or genital sinus. These compartments are either completely or partly cut off from the main coelom.

The coelom is filled with coelomic fluid which is similar to sea water in composition and contains usual coelomocytes. A few types of coelomocytes are recorded in echinoids. coelomocytes with pointed or broad pseudopodia are phagocytic in function and the types containing different coloured spherules have no phagocytic power.

The small and rounded variety with a vibratile flagellum is regarded to be dis-quamated cells from the coelomic epithelium. These cells lose flagella and become transformed into amoeboid coelomocytes. The relative frequency of these different coelomocytes also varies greatly in different forms.

5. Digestive System of Echinus:

The mouth is situated at the centre of the oral surface of the body and is encircled by a lip-like membrane, the peristome. The mouth leads into a buccal cavity from which originates the oesophagus. The alimentary canal is looped.

The oesophagus is slender and leads into the flattened stomach. A short caecum arises from the junction of oesophagus with the stomach. Extending from the beginning of the stomach up to the beginning of the intestine runs a slender tube, called siphon. The inner wall of the siphon is ciliated and a constant flow of water current passes from the anterior to the posterior direction of the siphon.

By the removal of excess water through the siphon the food matters become concentrated in the stomach for digestion. The intestine ascends as the narrow rectum which opens to the exterior through the anus on the aboral side.

In the digestive system of Echinus and in many sea-urchins there are many commensal ciliates and parasites. Besides them, many other commensals and parasites are recorded inside or outside the body.

Histological picture of the alimentary canal shows the following layers:

1. The outermost layer is covered by flagellated coelomic epithelium.

2. The next inner layer is composed of a very thin muscular layer (mostly cir­cular muscles).

3. Beneath the muscle layer lies a con­nective tissue layer.

4. The innermost layer lining the lumen of the alimentary canal comprises slender columnar ciliated epithelium. Gland cells are also present in this layer except in the intestine where the gland cells are reported to be wanting.

6. Respiratory System of Echinus:

The respiratory system of Echinus consists of five pairs of small, branched and thin-walled outgrowths of the body at the margin of the peristome. These are called peristomial gills or branchiae. Each inter-ambulacral area con­tains one pair of gills, one on each side.

The lumen of the gills is continuous with the peripharyngeal coelom. Besides these peristomial gills, five dental sacs are located on the top of the lantern of Aristotle. The tube-feet and the siphon help in respiratory process.

7. Water Vascular System of Echinus:

The water vascular system is fundamen­tally similar to that of Asterias. The main ring canal bears five small inter-radial Tiedemann’s bodies. The stone canal is membranous and is not calcified. Each radial canal gives a branch to the buccal podia and terminates as the lumen of the terminal tentacle. The axial sinus becomes enlarged under the madreporite to form an ampulla into which the stone canal opens.

8. Haemal System or Blood Lacunar System of Echinus:

The haemal system includes an oral hae­mal ring round the oesophagus and five radial haemal sinuses. The radial haemal sinuses end blindly at the aboral side.

9. Locomotion of Echinus:

Numerous elongated double rows of tube- feet, projecting from the ambulacral areas, are the principal locomotor organs. The mechanism of movement by the tube-feet is similar to that of Asterias. As because the arms are lacking in Echinus, the tube-feet become greatly extended towards the direc­tion of movement. Besides the tube-feet, the animal can also move by using the spines.

10. Excretory System of Echinus:

There is no definite excretory organ in Echinus. The nitrogenous wastes are ex­tracted and eliminated through the entire surface of the body.

11. Nervous System of Echinus:

The nervous system of Echinus consists of a nerve ring surrounding the mouth and five radial nerves which extend up to the terminal tentacles.

The radial nerves are en­closed by epineural canals and innervate the tube-feet, spines, pedicellariae, so-called ‘eye’ and sphaeridia. The circumoral nerve ring together with the radial nerves constitutes the ectoneural nervous system. The hyponeural and coelomic nervous systems are extremely ill-developed.

Sense organs:

The sense organs are not specialised. As stated earlier, the sphaeridia, distributed in the ambulacral areas, are the organs of bal­ance. There are five so-called ‘eyes’ (pigmented nervous cushions, one each ocu­lar plate nears the terminal tentacle) which are regarded as photoreceptors.

12. Reproductive System of Echinus:

Echinus is a dioecious echinoderm and sexual dimorphism is absent. The gonads are five in number. They are large, branched, racemose glandular masses in the perivis­ceral cavity. The Testes or ovaries are more or less similar in appearance and become voluminous at matured state. The gonads develop as the outgrowths of the genital rachis which becomes atrophied in adults.

Each gonad at its aboral end gives a short gonoduct which opens to the exterior through gonopore. There are five gonopores, one on each genital plate. After maturation, the gam­etes are set free in the sea water and the fertilization is external.