ECHINUS

(e-ki’nus), SEA-URCHIN, or SEA-EGG, a genus of marine animals the type of an order (Hchinoidea) of the phylum Echinoderm at a (sce above). In this type the body is spheroidal and invested in a test or shell composed of regularly arranged plates closely united together. It is covered with movable spines articulated by ball-and-socket joints. The mouth is situated in the centre of the under surface, and there is a complicated masticatory apparatus (Aristotle’s lantern) consisting of five chisel-ended teeth supported by an elaborate framework. The anus is similarly placed on the upper side, and is surrounded by a circlet of ter plates (apical disc), one of which bear, a furrowed tubercle (madreporite) perforated by small holes through which water enters the water-vascular system. Locomotion 'is effected by meridional rows of tube-feet, aided by the spines. HH. esculent us and some other species are edible. See SEA URCHIN.