An anatomical diagram of a hypothetical ancestral mollusc. Whether or not such a creature ever existed is a subject of debate among evolutionary biologists, but the construct or bauplan depicted here includes most of the physical attributes likely present in some form of shared molluscan ancestor, including a nervous system which is hypoathroid, an unsegmented, bilaterally symmetrical body, a single domed shell, and with organs occurring either singly or in pairs (i.e., with no metamerism).
Biologists began drawing bauplans of molluscs in the late 19th century in an attempt to summarize the common features of the molluscan phylum. As pictures, however, these illustrations sometimes may actually serve to constrain theoretical understanding rather than aid it, especially given the absence of any specific fossil evidence of such a creature having existed. Nevertheless, depictions continue to be made and revised: organs become rearranged, emphases shift and settle, and the pictures themselves become a subject of analysis (or meta-analysis) quite apart from their completeness or accuracy as scientific works.Diagram: K.D. Schroeder