The structure of the columellar muscle system in Clio pyramidata and Cymbulia peroni (Theocosomata, Gastropoda). With a note on the phylogeny of both species

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.