The following points highlight the ten representative types of Echinodermata. The types are: 1. Antedon 2. Holothuria 3. Cucumaria 4. Thyone 5. Echinus 6. Echinarachinus 7. Echinocardium 8. Astropecten 9. Pentaceros 10. Ophiothrix.
Echinodermata: Type # 1. Antedon:
Antedon (Fig. 86.1) is commonly known as feather star. Body consists of a central disc or calyx and a series of five radiating arms. Central disc or calyx is differentiated into an upper oral surface and lower aboral surface. Oral surface is covered with a soft and lethery skin, the tegmen bearing the central mouth and the anus on a papilla in an inter-radius.
Aboral surface bears several slender curved jointed cirri supported by small ossicles which serve for attachment. Each arm divided at its base into two, so that there are ten long slender flexible arms, bearing lateral pinnules. Five ambulacral ciliated grooves radiate from the mouth towards the arms, where each divides into two and the branches extend along the oral surface of the arms.
Tube feet or podia without suckers are present along the edges of ambulacral grooves. Sexes are separate, gonads contained in the dilated bases of pinnules. Development includes a pentacrinoid larva with jointed stalk. Antedon is marine, found at moderate depths attached to the rocks and stones.
Echinodermata: Type # 2. Holothuria:
Body of Holothuria (Fig. 86.2) is elongated, bilaterally symmetrical bearing the mouth and anus at opposite ends. Body bears numerous podia or tube feet, locomotory on the ventral surface and papilliate on the dorsal surface. Body wall is leathery having a skeleton of minute ossicles. Mouth is anteriorly placed, surrounded by 15-30 peltate tentacles. Respiratory tree is well developed.
Cuverian tubules are also present. Madreporite is internal. Sexes are separate, gonad single tuft attached to left side of the dorsal mesentery. Development includes auricularia larva. Holothuria feeds by pushing sand containing organic food into mouth with the help of tentacles. Holothuria is found in shallow tropical seas.
Echinodermata: Type # 3. Cucumaria:
Cucumaria (Fig. 86.3) is commonly known as sea cucumber. Body is elongated, cylindrical with mouth and anus at opposite ends. The body is five-side bearing five longitudinal ambulacra each provided with double rows of podia or tube feet. Oral end bears the large mouth surrounded by a circular lip and a thin peristomial membrane. Mouth bears ten highly dendritic or branched tentacles.
The smooth, thin and collar-like region at the base of tentacles is known as introvert. Anus is terminal and placed at the aboral end. Respiratory trees are present. Cuverian tubules are absent. Sexes are separate, gonads are arranged in two tufts. Development includes an auricularia larva. Cucumaria is marine and cosmopolitan in distribution.
Echinodermata: Type # 4. Thyone:
Body is elongated, somewhat swollen in the middle with mouth and anus at opposite ends. Podia or tube feet are distributed over the entire body surface. Oral end bears the mouth surrounded by 10 dendritic or branched tentacles. Two tentacles attached to the mid- ventral ambulacral area, are much smaller than others.
Anus is placed on the aboral end. Sexes are separate. Development includes auricularia larva. Thyone exhibits great power of regeneration and autotomy. Thyone is marine and found being completely buried in sandy and muddy bottom.
Echinodermata: Type # 5. Echinus:
Echinus (Fig. 86.5) is commonly known as sea urchin. Body is globular in shape, somewhat flattened at the two poles forming distinct oral and aboral poles. Body is enclosed in a rigid globular shell or corona formed of closely fitted calcareous plates. Entire surface of the animal except the peristome and periproct is covered with spines articulated to the shell.
Pedicellariae with three jaws and sphaeridia are present among the spines. Mouth lies in the centre of oral pole and is surrounded by soft membrane known as peristome, through the mouth project the five teeth of Aristotle’s lantern. At the aboral pole is a much smaller aperture, the anus surrounded by periproct.
The surface of the shell is divided into alternating ambulacral and inter-ambulacral areas. Numerous podia or tube feet project from the surface among the spines. These are arranged in five double rows in ambulacral areas. Sexes are separate, Gonads are five large masses. Development includes a free swimming echinopluteus larva. Echinus is marine, found in the sea in the rocky places.
Echinodermata: Type # 6. Echinarachinus:
Echinarachinus (Fig. 86.6) is commonly known as sand dollar. The body is almost circular disc-like and dorsoventraly flattened. The test is covered with numerous very small, soft and delicate spines.
Mouth is centrally placed, surrounded by five petaloid ambulacra. Peristome bearing the anus, lies on one edge. The aboral ambulacral areas are petaloid and with open ends. Echinarachinus is found buried in the sandy soil in the coastal line of North Atlantic and North Pacific Oceans.
Echinodermata: Type # 7. Echinocardium:
Echinocardium (Fig. 86.7) is commonly known as heart urchin. The body shape is heart-like and light brown in colour. Peristome is transversely elongated and mouth is eccentric and lies near the anterior end. The periproct having anus lies at the hinder end. Ambulacra are five, petaloid and aborally situated.
The anterior median ambulacrum is the largest and deepest bearing shorter but more numerous tube feet. Inter-ambulacral areas bear long, curved spines, while ambulacral areas bear only short, thin spines. Aristotle’s lantern and teeth are absent. Echinocardium is marine and found buried in sand at the depth of 10-15 cm. It is cosmopolitan in distribution.
Echinodermata: Type # 8. Astropecten:
Astropecten (Fig. 86.8) is a large, flat, five pointed sea star. The body is star-shaped, consisting of a central disc prolonged radially into five flexible triangular arms with tapering tips. The body is markedly flattened, with two distinct surfaces, a lower oral surface and an upper aboral surface.
On the oral surface, the mouth opens in the centre surrounded by a soft membranous area, the peristome. Five broadly opened ambulacral grooves extend from the mouth and pass each along the entire length of one of the arms. Tube feet project in two rows along each ambulacral groove.
The terminal tube foot or tentacle, which stands at the tip of each arm, has a pigmented spot, the eye at its base. The skin is soft and somewhat transparent, permitting the internal skeleton to be detected from outside. The skeleton consists of calcareous rods and plates which are embedded in the dermal layer and leave interspaces of soft tissue through which tube feet penetrate. Pedicellariae project around the spines.
On the aboral surface, near the periphery of the disc and opposite the angle between two arms, is a small circular grooved plate, the madreporite which is perforated by numerous pores which lead into the water vascular system. The anus is absent. The armature on the aboral surface consists of short blunt spines projecting in bundles known as paxillae.
The spines in each paxilla are arranged in a circle around a basal vertical stalk.
Very small soft finger-like dermal branchiae arise between the paxillae, specially on the disc, as hollow outgrowths from the skin. Astropecten is found in our seas from below the tide line to great depths. It creeps about very slowly on the sea- bottom, but at rest it lies partly buried in sand with the central part of its body raised into a cone above the sand.
Echinodermata: Type # 9. Pentaceros:
Pentaceros (Fig. 86.9) is not regarded as a separate genus, but it is now included in genus Oreaster. It is found commonly in the Indo-Pacific Ocean and around the West Indies; in the Bay of Bengal and Arabian Sea, it feeds on oysters, thus, it is very harmful to the pearl industry.
It has a diameter of about 25 cm, the body is very thick and regular star-shaped. It has a central disc and five tapering arms or rays, the arms are not sharply demarcated from the central disc, the disc is large and arms are short with broad bases, this shape is termed stellate.
The axes of arms are called radii, and the spaces between them are inter-radii.
The animal has a hard integument containing many calcareous plates or ossicles, echinoderms have the ability to use calcium carbonate from sea water to build their skeleton of ossicles. The two main surfaces are really lateral, but the upper surface is called aboral and the lower one is oral. The aboral surface is convex and oral surface is flat.
In the centre of the oral surface is a five sided aperture called actinostome which encloses a mouth surrounded by a membranous peristome. Radiating from the actinostome into the five arms are five narrow ambulacral grooves, bordering each one of these grooves are two rows of movable ambulacral spines on each side, the spines are protective and can close over the ambulacral grooves.
On each side of the ambulacral groove is a row of slender tube feet or podia, they are tubular with sucker-like ends, tube feet can be distended and fixed to the substratum for locomotion. At the end of each ambulacral groove is a bright red eye made of several ocelli, above the eye is a small non-retractile sensory tentacle which is olfactory, it is a modified tube foot with no ampulla.
Aboral surface is yellow to reddish-brown in colour and has many stout spines in irregular rows, there are large spines on the margins, all spines rest on ossicles. Between the spines of papulae (gills or dermal branchiae) project singly, they are made histologically of thinned body wall and their cavities are continuous with the coelom, they are retractile and function as respiratory and excretory organs.
Papulae are confined to the aboral surface only. In the centre is a minute anus, near the anus between two arms is a flat light coloured plate called madreporite lying asymmetrically. Near the papulae and around the spines on both oral and aboral surfaces are microscopic pedicellariae of two kinds, they are either stalked or sessile.
Echinodermata: Type # 10. Ophiothrix:
Ophiothrix (Fig. 86.10) is a common spiny brittle star. It has a small rounded central disc and five slender jointed arms arising from the lower surface of the disc. The arms are covered on all sides by the plates or shields and fringed with spines. The ambulacral grooves, dermal branchiae and pedicellariae are entirely absent. The oral surface bears a madreporite and a mouth possessing five movable plates serving as jaws.
The oral surface also bears five oral shields and podial pores. The base of each arm bears a pair of deep grooves, called bursal slits through which pass to outside the mature sex cells. The tube feet without suckers are present on the lower plates of arms. Ophiothrix possesses a great power of regenerating its lost arm. Ophiothrix is commonly found along Atlantic coast.