What Is a Snail?

Abstract

Fundamentally the body consists of two storeys connected by a waist. The lower storey, for locomotion and finding food, consists of a muscular foot and of a head, with senses and a mouth. The upper storey, for respiration, digestion, excretion and gamete production, also secretes the shell, which is attached to the snail by a well-developed muscle. Snails are also characterised by torsion: during embryonic development the visceral hump rotates 180° in relation to the foot; this huge change in the relationship between the major body parts is unique to snails in the entire animal kingdom.

The head has one pair of eyes, one or two pairs of tentacles, an abdomen used as a crawling organ, and a shell, which may grow as a broad cone or coil spirally. The foot carries a hardened lid, the operculum, to prevent predators from penetrating the shell and water from leaking out, in intertidal habitats. To crawl, power supplied by muscular contractions of the foot is transferred to the substratum through sticky mucus – an energetically very costly locomotion with very low efficiency.

The mantle cavity is usually a small area around the gills into which the viscera discharge their wastes. Digestive juices may contain cellulase – one of the very few cases throughout the animal kingdom of an animal producing its own decomposing enzyme.