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Prosobranchia was a large taxonomic subclass of , which included , and . This taxon dates back to the 1920s, but has since been proven to be polyphyly (consisting of more than one lineage of descent). In biology taxonomy must reflect phylogeny and monophyletic groups are preferred; in other words, the classification of a group must reflect its evolutionary descent as far as that is known through studies of Morphology (biology) and genetic analysis, and must include a common ancestor and all of its descendants. Therefore, the taxon Prosobranchia with its multiple evolutionary origins is no longer considered suitable to be used.
One can still encounter this subclass used as if it is still valid in many texts and websites. Although Prosobranchia is no longer generally accepted as a taxon by people who study living Mollusca, still the term prosobranch is legitimately used as an anatomically descriptive adjective or noun, and the taxon Prosobranchia is still sometimes used by paleontologists.
The majority of marine Gastropoda are prosobranch, as are a few land snails and freshwater snails. The prosobranch gastropods include the majority of marine snails, among them , conus, cowry, , , periwinkles, volutidae and , as well as numerous freshwater groups, and some land snails with an operculum. Prosobranchs have their , mantle cavity and anus situated in front of their heart. Most prosobranchs have Dioecy.
The nervous system of prosobranchs is twisted into a figure 8 due to a developmental process known as torsion. The eyes are situated at the base of the .
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