Phylum : Echinodermata
Greek words Ekina-spines & derma-skin
- They have body covered with spiny skin.
They are exclusively marine.
- Germ layers:
They are triploblastic containing 3 germ layers.
Adults are radially symmetrical and larval forms are bilaterally symmetrical.
- Body Grade:
They have organ-system grade of body organization.
They are coelomatic. Coelom is surrounded by ciliated peritoneum.
- Body division:
The body is unsegmented and variously shaped like globular, star-like, spherical, elongated, flattened or flower-like . Head is absent.
Head is absent.
They exhibit holozoic nutrition.
- Alimentary Canal:
Alimentary canal is straight and well-developed.
Digestion is extra-cellular inside the alimentary canal.
Respiration occurs by gills (branches) or papullaprotauding from the dermis, the tube feet and cloacal aperture.
- Circulatory System:
Blood vascular system is open type with fine radiating vessels.
The excretory organs are absent. Water vascular system helps in excretion.
- Nervous system:
There is lack of centralized nervous system.
- Body is streamlines, covered by smooth sleppary scale for aquatic adaptation.
- Heart is two chambered.
- They are poikilothermic.
- Respiration occurs by gills and gills are protected by operculum.
- They bear ten pairs if oranial nerves.
- Lateral line is present in the lateral size of body that helps in detecting water pressure.
- Swim bladder is present for hydrostatic function.
- Locomotion occurs by fins.
- Body is divided into head, trunk and tail. Tail is powerful prosulsive organ.