Central nervous system of Chaetoderma japonicum (Caudofoveata, Aplacophora): implications for diversified ganglionic plans in early molluscan evolution.

The organization of the central nervous system of an "aplacophoran" mollusc, Chaetoderma japonicum, is described as a means to understand a primitive condition in highly diversified molluscan animals. This histological and immunocytochemical study revealed that C. japonicum still retains a conservative molluscan tetra-neural plan similar to those of neomenioids, polyplacophorans, and tryblidiids. However, the ventral and lateral nerve cords of C. japonicum are obviously ganglionated to various degrees, and the cerebral cord-like ganglia display a lobular structure. The putative chemosensory networks are developed, being composed of sensory cells of the oral shield, eight precerebral ganglia, and eight neuropil compartments that form distinct masses of neurites. In the cerebral cord-like ganglia, three anterior, posterior, and dorsal lobes are distinguished with well-fasciculated tracts in their neuropils. Most neuronal somata are uniform in size, and no small globuli-like cell clusters are found; however, localized serotonin-like immunoreactivity and acetylated tubulin-containing tracts suggest the presence of functional subdivisions. These complicated morphological features may be adaptive structures related to the specialized foraminiferan food in muddy bottoms. Based on a comparative scheme in basal molluscan groups, we characterize an independent evolutionary process for the unique characters of the central nervous systems of chaetoderms.