Early development of the aplacophoran mollusc Chaetoderma

The early development of the trochophore larva of the aplacophoran Chaetoderma nitidulum (Mollusca: Caudofoveata = Chaetodermomorpha) is described using scanning and transmission electron microscopy and using fluorescence staining and confocal laser scanning microscopy of the muscle system. The most important features include the presence of a prototroch and a telotroch, serially arranged glandular cells (spiculoblasts) at the dorsal mantle, the occurrence of a ventral suture as a possible remnant of a former foot sole, and the presence of a pair of protonephridia. The development of the early anlagen of the circular body wall muscles does not show the anterior–posterior mode of formation that is typical for annelids, thus strengthening the hypothesis of a non‐segmented ancestry of Mollusca.