Cladogram showing possible evolutionary relationships among the molluscs
This figure shows one interpretation of molluscan phylogeny. Zoologsts believe that the lack of a shell in the class Aplacophora (a class that we didn't discuss) is an ancestral character. All other molluscs have a shell or are derived from shelled ancestors. The multipart shell distinguishes the Polyplacophora from other classes. Synapomorphies* that identify the various clades are shown, although a number of these have been modified or lost in some descendants. For example, shell coiling has been reduced or lost in many gastropods and cephalopods, and many gastropods have undergone detorsion. There are, of course, other interpretations of molluscan phylogeny. The extensive adaptive radiation (formation of new forms from an ancestral species) of this phylum has made higher taxomonic relationships difficult to determine.
* A synapomorphy is an apomorphy shared by two or more groups.