Asterias, the genus which includes the common starfish of the English coast (A. rubens), and this serves as a useful introduction to the study of the Echinodermata. It belongs to the class Asteroidea, and the order Cryptozonata. The animal consists of a central disc from which radiate several (usually five) arms. The mouth is at the centre of the lower or actinal side; it leads by a short oesophagus to a stomach from which two branches (hepatic caeca) run up each arm. The anus (not present in all Starfish) is on the centre of the upper or abactinal side. The main feature in the anatomy of the asteroids is the water vascular system; this consists of a ring round the mouth; on this ring are nine reservoirs (Polian vesicles), a canal which opens on the upper side by a filter-like plate (madreporite), and trunks, one of which runs up a furrow on the lower side of each arm; upon these are borne the tube feet by which locomotion is effected. The blood vascular and nervous systems consist of similar rings round the mouth, bearing a branch up each arm; the former has also a ring round the anus. The generative organs consist of a pair or pairs of glands in each arm. As each arm is thus provided with a complete set of organs, is bilaterally symmetrical in cross section, and is segmented, Hackel suggested that the starfish consisted of a series of worm-like animals fixed together by their heads. The Starfish live mainly on shellfish, and sometimes invade oyster beds in enormous numbers.