ADHESIVE PAPILLAE OF PHALLUSIA-MAMILLATA LARVAE - MORPHOLOGY AND INNERVATION

The swimming larvae of most solitary ascidians belonging to the Ascidi idae family bear three anterior, simple conic adhesive papillae. They secrete adhesive substances that are used to effect transitory settlem ent at the beginning of the metamorphosis. The adhesive papillae of ne wly hatched Phallusia mamillata larvae examined by the SEM are covered by the tunic. When the larvae are about to settle, the tunic becomes fenestrated over the central part of the papilla and bulb-ended microv illi protrude through the holes. These papillae have two types of elon gated cells: many peripheral cells and few larger central cells with m icrovilli and bundles of microtubules oriented along the major axis of the cells. We have done immunofluorescence experiments with an anti-b eta-tubulin monoclonal antibody (clone 2-28-33) reacting with axonal m icrotubules. Only the central cells of the papillae were stained and t he axons appeared to arise from the proximal ends of these cells. Thes e axons form a long nerve that reaches the brain vesicle. Branches of the same nerve appear to connect to the basal ends of the peripheral c ells. By confocal laser microscopy we were able to follow the course o f the papillary nerve. The two nerves connecting the dorsal papillae f use together into a single nerve that runs posteriorly. The nerve conn ecting the ventral papilla runs posteriorly for a long tract before fu sing with the nerve of the dorsal papillae just near the brain. The re ported observations raise the hypothesis that the central cells of the adhesive papillae might be primary sensory neurons and that they may have chemosensory function.