Anti-tubulin labeling reveals ampullary neuron ciliary bundles in opisthobranch larvae and a new putative neural structure associated with the apical ganglion.

This investigation examines tubulin labeling associated with the apical ganglion in a variety of planktotrophic and lecithotrophic opisthobranch larvae. Emphasis is on the ampullary neurons, in which ciliary bundles within the ampulla are strongly labeled. The larvae of all but one species have five ampullary neurons and their associated ciliary bundles. The anaspid Phyllaplysia taylori, a species with direct development and an encapsulated veliger stage, has only four ampullary neurons. The cilia-containing ampulla extends to the pretrochal surface via a long, narrow canal that opens to the external environment through a very small pore (0.1 μm diameter). Cilia within the canal were never observed to project beyond the opening of the apical pore. The ampullary canals extend toward and are grouped with the ciliary tuft cells and remain in this location as planktotrophic larvae feed and grow. If, as has been reported, the ciliary tuft is motile, the pores may be continually bathed in fresh seawater. Such a...