The name Limpet is used for many marine and freshwater gastropod species which have a simple conical shell . The phrase `` true limpets '' is used only for marine limpets in the ancient clade Patellogastropoda . The name limpet is perhaps most often applied to members of the clade Patellogastropoda , the true limpets , which are all marine ; however , the feature of a simple conical shell has arisen independently many times in gastropod evolution , in many different lineages .