Prosobranchia

a subclass of gastropod mollusks, sometimes divided into three independent subclasses. The shell is usually coiled and sometimes conical; very rarely is there no shell. An operculum that covers the mouth of the shell is usually situated on the dorsal side of the foot. The organs in the mantle cavity (gills, osphradia, anus, and openings of the kidneys and gonads) are displaced anteriorly; hence the gills are in front of the heart. The shell is from 1 mm to 60 cm long and weighs from fractions of a milligram to several kilograms.

Prosobranchs are found predominantly in seas and oceans, at even the most extreme depths. The majority are benthic; a few are planktonic or parasitic. Some species live in freshwaters, and others on land. There are about 15,000 extant species, assigned to several orders (from two to 24). The order Docoglossa, which is sometimes classified as the subclass Peribranchia, includes about 350 littoral marine species. Forms characterized by a symmetrical mantle cavity and by notches or slits in the shell are sometimes isolated in the subclass Aspidobranchia (about 450 marine species). These groups (together with lower Ctenobranchia) are often united in the order Diotocardia. The subclass Ctenobranchia, which is often considered as the order Monotocardia, is distinguished by the greatest diversity of species. Many prosobranchs are commercially valuable. Some are the intermediate hosts of parasites.