Pterapod_Diacria trispinosa_Thecosoma (Niebuhr, 1775) Phylum Mollusca / Class Gastropoda / Subclass Opisthobranchia / Order Thecosomata / Family Cavoliniidae, Opisthobranchs in the Suborder Euthecosomata, which includes the cavoliniids, all have a calcareous shell.