Sensory endings of the ascidian static organ (Chordata, Ascidiacea)

Summary

The ultrastructure of the static organ is examined in larvae of Diplosoma macdonaldi, a colonial ascidian, and Styela plicata, a solitary ascidian; the results are similar. As previous workers found, the cell body of a unicellular statocyte lies in the lumen of the sensory vesicle and contains the statolith. A narrow neck connects the cell body to an anchoring foot in the floor of the sensory vesicle. Two previously undescribed sensory endings project into the lumen just to the left of the statocyte, one anterior and one posterior to the neck. A network of fine processes from each ending contacts the statocyte body. It is proposed that movements of the statocyte cell body are detected by these endings. They arise from neurons in the ventral wall of the sensory vesicle that project axons to the visceral ganglion. The placement of the sensory endings may allow discrimination of the directon of statocyte deflection.