Chaetoderma

Chaetoderma is a genus of Gastropoda, remarkable for being either very primitive or degraded: It belongs to the group of Isopleura, of which Chiton is the best known form. It differs from that genus in four main points: the shell is represented only by a few spines or spicules; the mantle is reduced to a mere collar round the anus; the radula or complex armed tongue of other Gastropods is but a single tooth; the "ctenidia" or gills are represented by only one pair instead of a considerable number. These features are suggestive of degradation rather than of primitive structure.