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The phylogenetic relation between  Clio pyramidata  Linnaeus, 1767, and  Cymbulia peroni  De Blainville, 1818, has been studied with regard to the structure of their muscle systems. Specimens of both species collected from the North Atlantic Ocean and the Mediterranean were sectioned 5 μm thick and stained with Haematoxilin-Eosin, Crossmon or Azan for histological purposes or they were studied as cleared dissected or entire animals. The columellar muscle and subectodermal wing muscles of both species are described. It is concluded that Clio shows neoteny since it develops without metamorphosis. The asymmetry in anatomy proves that both  Clio  and  Cymbulia  descend from spiralised ancestors. For  Clio  two primitive characters are discussed which may be due to the “larval stage” of the adult but which also affirm the possible relation of molluscs to a coelenterate-like ancestor. Original structures found in  Cymbulia  are also discussed in the light of a possible phylogenetic relation between Thecosomata and Coelenterata, more in particular Conulata.

The Structure of the Columellar Muscle System in Clio Pyramidata and Cymbulia Peroni (Theocosomata, Gastropoda)