The interesting little Rhodope veranyii, which has no odontophore, has been associated by systematists both with these simplified Opisthobranchs and with Rhabdocoel Planarians.
The alimentary canal commences with the usual buccal mass; the lips are cartilaginous, but not armed with horny jaws, though these are common in other Opisthobranchs; the lingual ribbon is multidenticulate, and a pair of salivary glands pour in their secretion.
For a comparison of various Opisthobranchs, Aplysia will be found to present a convenient starting-point.
It is one of the more typical Opisthobranchs, that is to say, it belongs to the section Tectibranchia, but other members of the suborder, namely, Bulla and Actaeon (figs.
Opisthobranchs provided in the adult state with a shell and a mantle, except Runcina, Pleuro.
In the marine Streptoneura they are ectodermic projections which ultimately fall off; in the Opisthobranchs they are closed pouches; in Paludina and Bithynia they are canals as in Pulmonata.
Neomenia, mentioned first by Michael Sars in 1868 under the name Solenopus, was afterwards included among the Opisthobranchs by J.