Echinoderms are exclusively marine animals with radial symmetry and calcareous ossicles that make up their endoskeleton. They have pentamerous radial symmetry as adults but bilaterally symmetrical larvae. Their defining feature is an ambulacral water vascular system that uses tube feet for locomotion and helps filter feed. Echinoderms have both indirect development through larval stages and undergo metamorphosis from bilateral larvae to radially symmetrical adults.

1. PHYLUM ECHINODERMATA
• Exclusively marine ,presence of endoskeleton made up of calcareous ossicles.
• Adults are radially symmetrical but larvae are bilaterally symmetrical .
• Triploblastic , enterocoelous coelomates with pentamerous radial symmetry
having a calcareous endoskeleton of separate plates embedded in the skin .
• Presence of pedicellariae (capture and removal of debris )

2. • Many echinoderms bear spines and pincer like pedicellariae ,the spines are protective
in function ,the pedicellariae keep the body surface clear of debris and minute
organisms .
• Body segmented with globular ,star like spherical ,discoidal or elongated shape ,body
surface is marked by five symmetrical radiating areas (ambulacra) and five alternating
inter radii (inter ambulacra)

3. • Body wall consists of an outer epidermis (single layered and ciliated ) , a middle
dermis and an inner lining of peritoneum .
• Alimentary canal is straight or coiled .
• Respiratory organs include dermal branchiae (eg ., star fish).tube feet ,respiratory
tree (e.g., Holothuria ), bursae (brittle star ) and peristomial gills system .

4. • Presence of ambulacral system or water vascular system is the most characteristic
feature .A perforated plate called madreporite ,allows water into the system
,water vascular system is of coelomic origin .
• Tube feet help in locomotion .
• The circulatory system is greatly reduced and is of the open type , it is called the
hemal system , blood often lacks a respiratory pigment ,there is no heart.

5. • Nitrogenous waste diffuses out via gills .
• The nervous system includes a nerve ring and radial nerve cords ,there is no brain .
• Poorly developed sense organs include tactile organs ,chemoreceptors ,terminal
tentacles ,photoreceptors and statocysts.
• Reproduction is both asexual and sexual .sexes are usually separate with a few
exceptions ,copulation does not occur ,fertilization is external .

6. • Development is indirect through free swimming larval forms .the bilateral symmetry
larvae undergo metamorphosis to change into radially symmetrical adults .
• Echinoderms resemble chordates in early embryonic development .
Examples ; Asterias (star fish),Echinus (sea urchin ), Antedon
(sea lily), Cucumaria (sea cucumber ,Ophiura( brittle star)