Neurogenesis in the mossy chiton, Mopalia muscosa (Gould) (Polyplacophora): evidence against molluscan metamerism.

TLDR: Outgroup comparison with all other molluscan classes and related phyla suggests that the cord‐like, nonganglionized cerebral system in the Polyplacophora is a reduced condition rather than a primitive mollUScan condition.

Abstract: Neurogenesis in the chiton Mopalia mus- cosa (Gould, 1846) was investigated by applying differen- tial interference contrast microscopy, semithin serial sec- tioning combined with reconstruction techniques, as well as confocal laser scanning microscopy for the detection of fluorescence-conjugated antibodies against serotonin and FMRFamide. The ontogeny of serotonergic nervous struc- tures starts with cells of the apical organ followed by those of the cerebral commissure, whereas the serotonergic pro- totroch innervation, pedal system, and the lateral cords develop later. In addition, there are eight symmetrically arranged serotonergic sensory cells in the dorsal pretro- chal area of the larva. FMRFamide-positive neural ele- ments include the cerebral commissure, specific "ampul- lary" sensory cells in the pretrochal region, as well as the larval lateral and pedal system. In the early juvenile the cerebral system no longer stains with either of the two antibodies and the pedal system lacks anti-FMRFamide immunoreactivity. Outgroup comparison with all other molluscan classes and related phyla suggests that the cord-like, nonganglionized cerebral system in the Polypla- cophora is a reduced condition rather than a primitive molluscan condition. The immunosensitivity of the pedal commissures develops from posterior to anterior, suggest- ing independent serial repetition rather than annelid-like conditions and there is no trace of true segmentation during nervous system development. Polyplacophoran neurogenesis and all other available data on the subject contradict the idea of a segmented molluscan stem spe- cies. J. Morphol. 253:109 -117, 2002. © 2002 Wiley-Liss, Inc.