Prosobranchia mollusc shell. Coloured scanning electron micrograph (SEM) of the shell of a prosobranchia mollusc. The prosobranch molluscs are a type of gastropod (snail), and most of them are found in the sea (marine gastropods). Like all gastropods, most prosobranchs have spiral-shaped shells. Common prosobranchs include conches, limpets, periwinkles, and whelks. The shell is typically formed from a calcium-based mineral. It protects the soft body of the mollusc inside. The successive spirals grow as the mollusc increases in size. The opening of the shell is at lower left.