Phylum Echinodermata

Phylum Echinodermata (Greek echinus = spine, derma = skin) includes animals such as Asterias (sea star), Cucumaria (sea cucumber), Echinus (sea urchin), Antedon (sea lily) and Ophiothrix (brittle star). These are exclusively marine animals that may be solitary, sedentary, free-living, or found in large groups (gregarious), typically living on the sea floor (benthic).

Echinoderms are radially symmetrical, with a distinctive pentamerous (five-part) symmetry in the adult form. The body may be spherical, elongated, or star-shaped, and lacks well-defined body divisions; instead, the two surfaces of the body are described as oral (bearing the mouth) and aboral (opposite the mouth). The endoskeleton is made of calcareous plates called ossicles, and the body surface bears spines, which give the phylum its name.

A unique and defining feature is the water vascular system, through which water enters via a sieve-like structure called the madreporite; this hydraulic system is used for locomotion, capturing food, and respiration. The digestive system is complete, with the mouth located on the oral surface and the anus on the aboral surface.

Respiration occurs through structures such as peristomial gills, papillae, or the respiratory tree, depending on the group. Echinoderms lack distinct circulatory and excretory systems. The nervous system is simple, consisting of a nerve ring around the mouth with radial nerves extending into the arms. …