Definition and Indroduction
Bilateral and deuterostomial eucoelomate eumetazoa with rough , tough and leathery skin due to a dermal skeleton of usually spiny ,calcareous ossicles, and the basic bilateral symmetry of larva modified in the adult, to a pentamerous radial symmetry .
The terms Echinodermata means spiny skin (Gr. Echinos= spiny + dermatos =skin) About 6,000 living and 20,000 extinct species known.
Although Jacob klein (1738) had earlier coined the name Echinoermata yet Linnaeus included these animals under Mollusca and Lamarck under his class radiate as echinoderms .Finally leuckart (1847) raised the group to the status of a separate phylum.
1. All species marine sessile, creeping or crawling at sea bottom,. Rarely free swimming .
2. Body mostly of moderate size, spherical, cylindrical or star shaped triploblastic and unsegmented .
3. Symmetry bilateral in larvae but pentamerous radial in adults .
4. No distinct head or bagin .
5. Oral surface distinguished into 5 radial ambulacral areas alternating with interambulacral areas around an oral aboral axis .
6. Body wall rough, tough and leathery due to mesodermal calcareous ossicles and spins ( internal skeleton) covered by epidermis which is usually ciliated .
7. Body cavity is a true enterocoelic coelom surrounded by ciliated peritoneum ,and divided into several tubular spaces and sinuses. The latter from 3 specialized system ,viz water vadcular, haemal and perihaemal system.
8. Locomotion by special small and hollow, sac like tubular processes , called tubefeet, and connected with water vascular system .Tubefeet also assist in feeding sensory reception and respiration .
9. Haemal and perihaemal system are connected with blood circulation.
10. Digestive system simple. Alimentary canal usually coiled. No anus or intestine in some.
11. No organs for excretion. Certain respiratory organs may be present .
12. A diffused and uncentralized nervous system and less developed sensory organs.
13. Reproduction mainly sexual sexes separate extensive power of regeneration .
14. Fertilization external in water. Life cycle includes bilateral, ciliated larval stages which metamorphose into adults.