ABSTRACT
This chapter introduces the aplacophorans, polyplacophorans, and monoplacophorans, loosely called the ‘placophoran’ groups, which have been regarded as the most ‘primitive’ living members of the molluscan tree, although not without controversy. Polyplacophorans, or chitons, are small to rather large, entirely marine, dorsoventrally flattened animals that are elongate-oval in shape. Polyplacophorans are ecologically important as they are often abundant in the littoral zone and, from the evolutionary viewpoint, are sometimes considered the most primitive living molluscs. Despite attempts to resolve molluscan phylogeny using morphological and molecular data, there is no consensus view regarding the position of Polyplacophora within the molluscan tree, in part due to poor sampling for basal taxa and the considerable extinction that has occurred. K. G. Wingstrand noted their similarity to those in monoplacophorans, stating that the ‘entire radula [sic] apparatus of the Polyplacophora shows so many features identical to those of the tryblidians that a derivation from a similarly shaped apparatus in a common ancestor appears unavoidable’.
