Fig. 1

Dorso-ventral and antero-posterior body axes in adult mollusks. Dorsal (d)–ventral (v), anterior (a)-posterior (p) axes indicate the orientation. The (anlagen of the) cerebral ganglia/commissure (blue) are located anteriorly, while the statocysts (sc) are located posteriorly (statocysts are absent in polyplacophorans). The trajectory of the digestive tract is lined in red. a An adult polyplacophoran with a pronounced antero-posterior axis, a dorsal shell (sh) and ventral mouth (mo) and foot (f). b The mouth and foot of the adult scaphopod Antalis entalis are located ventrally, while the scaphopod pavilion (pv) lies dorsally. c The mouth and brachial crown (brc) of the adult cephalopod Idiosepius notoides are located ventrally, while the mantle apex (ma) is located dorsally. an anus, fn funnel, m mantle and pt prototroch