- The term Echinodermata (Gr., echinos= spiny, derma=skin), was coined by Jacob Klein
- Echinoderms are triploblastic, radially symmetrical (adult) often pentamerous, larva with bilateral symmetry, sessile or creeping at bottoms.
- Echinoderms are exclusively marine deuterostomes.
- All echinoderms are benthic ; bottom dwellers (except few pelagic sea cucumbers), mostly free living except sea lilies.
- Spiny skinned; enterocoelic coelomate; organ system grade of organization; gregarious
- Body is unsegmented
- Echinoderms are uncephalized (head absent), body parts organized about an oral-aboral axis.
- Endoskeleton (mesodermal origin) is composed of unique calcareous ossicle bearing protective spines.
- Digestive tract is complete except Brittle star.
- An outstanding characteristics is echinoderm is their complex coelom (enterocoel) and its ambulacral system, derivatives; the water vascular system , perivisceral coelom and haemal system.
- Locomotion by contractile appendages of ambulacral system/ water vascular system called tube feet.
- Tube feet consists of ampulla, podium and sucker. It also serves for respiration and food capture.
- Presence of pinna like structure called pedicellariae that protects the skin gills.
- Excretory system is absent.
- Nervous system is diffused decentralized, usually consisting of nerve net, nerve ring and radial nerves, but no brain.
- Sense organs are poorly developed and consists of tactile tentacles, pigment eye spots and statocysts.
- Respiration by Dermal branchiae or papulae (star fish), gills (sea urchin), genital bursae(brittle star), cloacal respiratory tree (sea cucumber).
- Circulatory system is open type : heart absent. Blood does not contain respiratory pigment.
- Mostly dioecious, copulation absent, fertilization external.
- Eggs are homolecithal (yolk uniformly distributed in egg)
- Cleavage is radial and indeterminate
- Development is indirect through free swimming larval stages.
