Phylum : Echinodermata
Greek words Ekina-spines & derma-skin 1. They have body covered with spiny skin. 2. Habitat: They are exclusively marine. 3. Germ layers: They are triploblastic containing 3 germ layers. 4. Symmetry: Adults are radially symmetrical and larval forms are bilaterally symmetrical. 5. Body Grade: They have organ-system grade of body organization. 6. Coelom: They are coelomatic. Coelom is surrounded by ciliated peritoneum. 7. Body division: The body is unsegmented and variously shaped like globular, star-like, spherical, elongated, flattened or flower-like . Head is absent. 8. Cephalisation: Head is absent. 9. Nutrition: They exhibit holozoic nutrition. 10. Alimentary Canal: Alimentary canal is straight and well-developed. 11. Digestion: Digestion is extra-cellular inside the alimentary canal. 12. Respiration: Respiration occurs by gills (branches) or papullaprotauding from the dermis, the tube feet and cloacal aperture. 13. Circulatory System: Blood vascular system is open type with fine radiating vessels. 14. Excretion: The excretory organs are absent. Water vascular system helps in excretion. 15. 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.