CLASSIFICATION OF ECHINODERMS
The Echinoderms are triploblastic, coelomate animals. They have pentaradial symmetry in adults. They possess a water-vascular system. Their endoskeleton is covered by epithellium. Pedicellariae are present. Phylum Echinodermata has following classes:
1. Class Crinoidea
They are free living or they are attached by an aboral stalk of ossicles. They flourished in the Paleozoic era. This class has 230 living species.
Examples: Sea lilies, feather stars
2. Class Asteroidea
Rays or arm are not sharply arised from central disk in Asteroidea. Ambulacral grooves are present with in tube feet. The tube feet have suction disks. Pedicellariae are present in them. They have about 1,500 species.
Example: Sea stars.
3. Class Ophiuroidea
Their arms sharply marked off from the central disk. Their tube feet are without suction disks. They have 2, 000 species.
Example: Brittle stars.
4. Class Concentricycloidea
They have two concentric water vascular rings. These rings encircle a disk like body. They have no digestive system. They digest and absorb nutrient from their lower surface. They have internal brood pouches. ‘Hwy do not develop free swimming larval stage.
Example: Sea daisies
5. Class Echinoidea
Their body is globular or disk shaped. They have no rays or arms. They have movable spines. Their skeleton is composed of closely fitting plates. They have 1, 000 species.
Examples: Sea urchins, sand dollars
6. Class Holothuroidea
They have no rays. They are elongated along the oral-aboral axis. Microscopic ossicles are embedded in a muscular body wall. They have circumsoral tentacles. They have 1, 500 species.
Example: Sea cucumbers.