THE Gastropods are specially characterized, firstly, by their asymmetrical organization ; secondly, by their well-developed head ; and, thirdly, by their shell, which is formed of one piece, and coiled in a spiral, at least in the larval stage. The asymmetry of some of the principal organs of the body is the chief characteristic of the Gastropoda. The essential feature of this asymmetry is that the anus generally lies to one side of the median plane ; that the ctenidium (gill-combs), the osphradium (olfactory organs), the hypobranchial gland (or pallial mucous gland), and the auricle of the heart are single, or at least are more developed on one side of the body than the other ; and that there is only one genital orifice, which lies on the same side of the body as the anus.
The diet of Gastropoda varies according to the group under consideration. Generally speaking, the carnivorous habit is due to specialisation ; various forms live and feed on colonial invertebrates, such as Hydrozoa. Some Gastropoda are parasitic, generally in or upon Echinoderms.