ABSTRACT
Aristotle recognised two groups of molluscs: Ostrachodermata for those with shells and Malachia for the cephalopods. Many treatments of molluscan phylogeny have used archetypes, Bauplane, or other images of reconstructed common ancestors to postulate what the first mollusc looked like. Attempts to define the molluscan ancestor also depended on ideas about what the sister taxon of the Mollusca is. Concepts of ancestral molluscs thus included designs based on shared ancestry with turbellarian flatworms, reduced annelids, or even entoprocts. The grouping of molluscan classes into higher taxon groups has been a popular pastime among molluscan workers, with several names proposed for those groupings. There have been several alternative hypotheses of the relationships of the major molluscan groups to one another that have been derived from analyses involving morphological, molecular, developmental, and fossil data. Most workers consider living molluscs to be divisible into two groups, the Aculifera and the Conchifera.
