The salient features crinoidea are the following 1.Body is stalked or stalkless and pentamerous and consist of an aboral cup and a oral cover. The cup is called calyps or disc, and the cover is called Tegmen. 2.Mouth and anus are located on the upper side. 3.Arms are long and movable,branched or unbranched, and with or without alternating bilateral processes, called Pinnules. 4.Ambulacral grooves are open and ciliated and they extend up to the tip of arms and pinnules. 5.Madreporite, spines and pedicellariae are absent. 6.Tube feet have no suckers. 7.Sessile forms have aboral, jointed and heavily calcified stalk for attachment. 8.Sexes are separate, developments is indirect with a barrel shaped larava called doliolaria. Eg:- Antedon, Diplocrinus, Metacrinus, Rhizocrinus.
Asteroids are flattened and free living echinoderms, commonly called Star fishes or sea stars.Their salient features are the following: 1.Spiny, flat and pentaggonal body with a central disc, 5-20 or more radial arms or rays, and distinct oral and aboral surfaces. 2.Mouth and ambulacral grooves are on the lower side, and annus and madreporite are on the upper side. 3.Ambulacral grooves are open channels, studded with sucker –bearing tube feet. 4.Dermal ossicles are loose, flexible and separate pieces. 5.Respiratory structures are dermal branchiae or skin gills. 6.Sexes are separate. Development is indirect with bipinnaria and brachiolaria larvae. There are nearly 1500 species of living asteroids. They enjoy worldwide distribution, especially in coastal waters. EG:-Asterias, Asteropecten, Pentaceros, Solaster, Echinaster.
Ophiuroidea comprises the echinoderms, popularly known as basket stars, brittle stars, or serpent stars. The major characteristics of ophiuroids are following: 1.Flat body, with a thin disc and five or more slender and flexible arms. 2.Arms are formed of a series of segment-like sections or articles, each article being formed of four shields. 3.Ambulacral grooves, pedicellariae and dermal branchiae are absent. 4.Tube feet are without ampullae and suckers. 5.Gut is a blind sac, without intestine and anus. 6.Madreporite is on the oral surface. 7.Sexes are separate.development is indirect with a lava called ophiopluteus. 8.Many ophiuroids have the powers for autotomy (self multilation) and regeneration. Ophiuroidea is the largest class of echinoderms with nearly 2000 described species. EG:- ophiothrix, Ophiocoma, Ophioderma, Ophiolepis, Gorgonocephalus
Holothuroidea is a group of leathery and almost cylindrical echinoderms, commonly called sea cucumbers. The major diagnostic features of holothurroids are the following: 1.Body is long, cylindrical and almost bilaterally symmetrical. 2.Body wall is leathery and dermomuscular. 3.Mouth and anus are at opposite ends. Mouth is encircled by crown of retractile tube feet, known as buccal podia, which are modified as oral tentacles. 4.Radial arms, spines and pedicellariae are absent. 5.Dermal ossicles are microscopic, vestigial and loosely scattered. 6.Ambulacral grooves are absent and madreporite is internal. In most cases, madreporic canals have no external openings or hydropores. 7.Alimentary canal iss long and coiled, with definitee cloaca. 8.Connected to the cloaca are the so-called respiratory trees or cloaca gills. 9.Circulatory system is more advanced than that of other echinoderms, and it functions to transport oxygen, and perhaps nutrients also. 10.Development is indirect with characteristic larva, called auricularia. EG:-Holothuria,Mesothuria, Cucumaria, Thyone, Synapta, Synaptula.
Echinoidea includes echinoderms popularly called sea urchins, heart urchins, cake urchins,and sand dollars. There are 950 species of known echinoids. The salient features of echinoidea are the following: 1.Body is globular, oval, discoidal, or heart shaped with definite oral and aboral surfaces. 2.Arms and ambulacral grooves are absent. 3.The whole body is enclosed in a box-like shell, known as corona or test. it is formed of calcareous plates. 4.Borne on the test, and covering the whole body, is an armature of movable spines. 5.Mouth is surrounded by leathery membrane, called peristome,and anus by periproct. 6.Mouth is armed with a characteristic chewing apparatus called Aristotle’s lantern. 7.Arranged in between the spines and extending between peristome andd periproct, are five double rows of tube feet. 8.Sexes are separate.development is indirect with a characteristic larva, called echinopluteus. EG:-Salmacis, Echinus, Echinocardium, Echinodiscus, Clypeastear.