Members of the phylum Echinodermata are exclusively marine and most of them are bottom dwellers. The most striking characteristic of most species of this group is the pentamerous radial symmetry. This symmetry, however, has been secondarily derived from a bilateral ancestral form and is lost in some recent species. Another characteristic, unique to the echinoderms, is the water-vascular, or ambulacral, system.
The phylum is divided into five easily recognized classes:
1. Crinoidea, the stalked sea-lilies and feather-stars (Greek: 'lily-like'). This class is most primitive and comprises most sessile species as well as a number of free-swimming forms without a stalk.
2. Asteroidea, the starfishes (or sea-stars) and cushion-stars (Greek: 'star-like').
3. Ophiuroidea, the brittle-stars and basket-stars (Greek: 'snake-like').
4. Echinoidea, the sea-urchins, sand-dollars and heart-urchins (Greek: 'spiny').
5. Holothuroidea, the sea-cucumbers (Greek: 'sea-cucumber').