Members of Echinoderms

The following points highlight the nineteen interesting members of Echinoderms. The members are: 1. Astropecten 2. Heliaster 3. Zoroaster 4. Ctenidiscus 5. Porcellanaster 6. Luidia 7. Solaster 8. Clypeastre 9. Diadema 10. Arbacia 11. Laganum 12. Echinocardium 13. Thyone 14. Cucumaria 15. Pelagothuria 16. Opheodesoma 17. Ophioderma 18. Gorgonocephalus 19. Metacrinus.

Echinoderms: Member # 1. Astropecten:

Astroyecten (Fig, 21. 26) is a widely distrib­uted starfish. They inhabit sandy bottom of sea where they can burrow. Astropecten polycanthus remains buried in sand most of the time and come out twice a day in search of food—once in the morning and once in the evening.

The genus Astropecten is con­sisted of about 100 species. The body con­sists of a flattened five-pointed disc with short arms. There are rows of spines border­ing the arms (Fig. 21.26).

The inframarginal plates are elongated and meet the adambulacrals to form the oral surface of the arms. The pedicellariae are either sessile or pectinate types. The conical tube-feet are devoid of suckers and possess circular mus­cles. The ampullae are bifurcated. The speed of locomotion is recorded in some forms of this genus.

Astropecten auranciacus moves 30- 60 cm per minute and Astropecten spinulosus moves 60 cm per minute. The mouth is very wide, distensible and can swallow the prey (bivalves, snails, crustaceans, annelids) very easily. The stone canal is extremely compli­cated due to extensive development of the internal ridge.

The ridge divides the stone canal into two tubes, each of which contains a pair of scrolls. Two to four polian vesicles are given in each inter-radius. These vesicles emerge from a common stalk (Fig. 21.38). The optic cushion lacks pigment cup and the retinal cells are distributed throughout the optic cushion. The brachiolaria larva stage is absent.

Echinoderms: Member # 2. Heliaster:

Heliaster (Fig. 21.27A) is the single genus of the family, Heliasteridae. The genus con­tains a few species and lives in shallow water of the Panamic region.

The body consists of a very broad disc with numerous short ta­pering arms. The number of the arms varies from 20 to 44. Clark (1907) has shown that Heliaster starts its life with five arms and additional arms are added in all the inter-radii excepting the inter-radius containing the madreporite.

Only one madreporite is present. The skeleton is of reticulate type. The pedicellariae are straight and crossed types. The disc coelom and the brachial coelom are almost separated by a circular vertical wall with which the inter-brachial septa are joined by their inner ends.

Echinoderms: Member # 3. Zoroaster:

Zoroaster (Fig. 21.27D) is the typical genus of the family Zoroasteridae. It inhabits the deep sea. The body consists of a small disc with five elongated slender arms. The pedicellariae are of straight type. The aboral side of the body contains small spines with small papular zones. Usually four rows of tube-feet are present in each ambulacral groove.

Echinoderms: Member # 4. Ctenodiscus:

Ctenodiscus (Fig. 21.27B) is a typical genus under the family Goniopectinidae. The members of the genus are generally mud-dwellers. The body is stellate-shaped and the marginal plates are very prominent. The cribiform organs (a cavity containing lamellae and bounded by marginal plates) are very simple in organisation. The alimen­tary canal lacks intestine, intestinal caeca and anus. An epiproctal cone is present.

Echinoderms: Member # 5. Porcellanaster:

Pdrcellanaster (Fig.-21.28C) is the typical example of the family Porcellanasteridae. The representatives of the genus are mud- dwellers and live at considerable depths. The body consists of star-shaped disc and five narrow pointed arms. The arms are bordered by thin marginal plates.

The aboral side is membranous with a central elevation, called epiproctal cone. The alimentary canal lacks intestine, intestinal caeca and anus. The tube-feet are pointed and lack suckers. The ampullae are simple. In each inter-radius single cribriform organ is present.

Echinoderms: Member # 6. Luidia:

Luidia is the only genus under the family Luididae and has about 42 species. This ge­nus is distributed in the tropical and subtropi­cal regions.

The body consists of a very small disc with elongated flexible arms. Most of the species possess five arms and others may have six to eleven arms. The arms are bor­dered by spines. The papulae are branched. The anus and the intestinal caeca are absent. The pedicellariae are usually present.

Echinoderms: Member # 7. Solaster:

Solaster (Fig. 21.27E) is a typical multi- rayed sun-star belonging to the family Solasteridae. The body consists of a very broad central disc and 7-14 short tapering arms. The aboral skeleton is reticulate type and contains groups of small spines.

Echinoderms: Member # 8. Clypeaster:

Clypeaster (Fig. 21.27G) is the only surviv­ing genus of the family Clypeasteridae. They are distributed in the tropical and subtropi­cal regions of the earth and inhabit the sandy bottom. They are commonly called sand- dollars. The test is more or less round in outline and is covered with thick short spines.

The madreporite is placed at the centre of the aboral side from which radiate five petaloid ambulacral areas. There are five simple grooves along the centre of the ambulacral areas.

At the beginning of each ambulacral area near the peristome there exist two sphaeridia. The sphaeridia are devoid of nerve ring and are not movable. The genital plates are fused with the central pentagonal plate and the gonopores are present at the angles of the pentagon.

Echinoderms: Member # 9. Diadema (Centrechinus):

Diadema (Fig. 21.27F) is a typical genus under the family Diadematidae. It lives in littoral and sub-littoral zones of the tropical and subtropical regions. The common Indo- Pacific species of the genus are Diadema setosum and Diadema savignyi. Diadema is very large in size with long primary spines.

These spines may reach a length of 30 cm. The spines are very sharply pointed. The test is inflexible and the aboral inter-ambulacral areas lack spines. The periproct is devoid of skeletal elements and possesses a prominent anal cone. The globiferous pedicellariae are absent.

Echinoderms: Member # 10. Arbacia:

Arbacia (Fig. 21.27C) belongs to the fam­ily Arbaciidae. They usually live in the lit­toral zones. The peculiarities of the genus are the presence of the primary spines and the possession of one sphaeridium in each ambulacrum at the oral end.

The sphaeridia are placed in pit-like cavities distributed along the centre of the ambulacra. The periproct contains usually four or five plates which act as valves for the centrally placed anus. The primary spines are of moderate size and the secondary spines are absent.

Echinoderms: Member # 11. Laganum:

Laganum (Fig. 21.28A) belongs to the fam­ily Laganidae. They inhabit the sandy bot­toms in shallow water of Indo-West Pacific region. The test is small, flattened and circu­lar in outline. The ambulacra at the aboral side become petaloid.

The apical skeletal elements are fused into a central pentagonal plate containing five gonopores. The madreporic pores usually sink into pits. The oral surface bears five ambulacral grooves not reaching the edge of the test. The periproct is situated on the oral surface.

Echinoderms: Member # 12. Echinocardium:

Echinocardium (Fig. 21.28E) is the typical Heart Urchin under the family Loveniidae. The members under the genus enjoy cosmo­politan distribution. They bury in sand. The test is large in size with four well-developed petaloids. On the aboral side, the large spine tubercules with deeply sunken areoles, so common in other genera, are wanting.

The apical central plate contains four gonopores. The peristome becomes transversely extended. The lantern of Aristotle is absent. The ambulacra bear numerous short tube- feet and short spines. The inter-ambulacra bear long spines.

Echinoderms: Member # 13. Thyone:

Thyone (Fig. 21.28D) is a typical genus under the family Cucumaridae. It is found to remain buried in sandy and muddy bot­toms. The body is more or less oval with ten branched tubules. The mid-ventral pair of the tentacles is smaller than the others. The tube-feet are present all over the body with­out having any specific orientation in the ambulacral areas.

The respiratory trees are present, but the Cuverian tubules are ab­sent. The haemal system is very simple. The haemocytes in the coelomic fluid contain haemoglobin. The stomach is an elongated muscular sac of limited length and not very extensive as in other holothurians.

Echinoderms: Member # 14. Cucumaria:

Cucumaria belongs to the family Cucumaridae (Fig. 21.29E). At the oral end there are ten tentacles. The tube-feet are distributed as five distinct ambulacral bands. In the inter-ambulacral areas tube-feet are a few in number and are haphazardly distrib­uted.

In Cucumaria frondosa, numerous (about 50) male gonopores are present. The haemocytes in the coelomic fluid also con­tain haemoglobin. The complicated lacunar network’s connecting the intestine is absent. They are hermaphroditic.

Echinoderms: Member # 15. Pelagothuria:

Pelagothuria (Fig. 21.28B) is a typical elasipod genus. It is a floating form, ranging from the surface to considerable depths. It has an oval body without ossicles. The mouth is wide and terminal. It is surrounded by a circlet of tentacles with bifurcated tips. The number of tentacles varies from 13-20 in different species.

A floating sail supported by many long papillae is present just behind the circlet of tentacles. This sail may be restricted to the dorsal side or may encircle the body. The tube-feet are ill-developed. The stone canal opens to the exterior near the genital papillae.

Echinoderms: Member # 16. Opheodesoma:

Opheodesoma (Fig. 21.29) belongs to the family Synaptidae. It lives completely in buried condition in muddy and sandy bot­toms of the sea. It has an elongated vermi­form body which may reach the length of about two to three feet.

About fifteen well- developed pinnate tentacles are present. The tube-feet are absent. The respiratory trees are absent and the thin body wall serves as the respiratory surface. The body wall contains anchor plates. The sigmoid ossicles are lacking.

Echinoderms: Member # 17. Ophioderma:

Ophioderma is a well-known representa­tive of the family Ophiodematidae. The body consists of a small pentagonal disc covered by closely set granules. The oral papillae are numerous and are arranged in a continuous series.

The teeth are present in a single row and the dental papillae are absent. The spines of the arms are small. The most important individual characteristic feature of the genus is the presence of two slits in each bursa, one situated orally and the other peripherally.

Echinoderms: Member # 18. Gorgonocephalus (Basket-star):

Gorgonocephalus is the best known genus of the family Gorgonocephalidae. The body consists of a large disc and many elongated extensively branched flexible tentacles (Fig. 21.28F). Gorgonocephalus starts life with five arms which become repeatedly branched in advanced stage. The oral surface of the arms is spiny and the aboral surface is annulated due to lack of true aboral arm shields.

The lateral and oral arm shields are present. The margin of the disc between the radial shields is provided with series of plates. The oral papillae and tooth papillae are present. The gonads are restricted to the interior of the disc. The bursae become fused to form large cavities which suppress the coelom.

Echinoderms: Member # 19. Metacrinus:

Metacrinus is a very common pentacrinite genus of the family Isocrinidae. This genus is distributed in the Japan-Malay-Australia regions. The stalk is long pentagonal and with many joints or nodes. Encircling the nodes, there are whorls of five many-jointed cirri for temporary attachment.

The arms are much branched, pinnulated and arranged like a crown of flower. The most important characteristic of the genus is the existence of four brachial ossicles between the radials and the first branching. All the forks of the arms are separated by a number of brachials.