A large group of marine, land, and freshwater gastropods (helically coiled snails, slugs, and limpets within the phylum Mollusca) that is characterised literally by the presence of "gills in front of the heart." The Prosobranchia grouping includes the large majority of gastropods (~60% of living genera), many of which are commonly recognised sea shells (see illustration), and is incredibly diverse in terms of anatomy, life history, and ecology. Prosobranchs exhibit an enormous array of life modes (free-living crawlers, burrowers, floaters, active swimmers, sedentary cemented forms) and have conquered almost every marine, freshwater, and terrestrial habitat. They are primarily marine, but estimates suggest that there have been as many as 33 colonizations of fresh (continental) waters; a number of families are amphibious or terrestrial, but these are all dependent upon water to varying degrees.