Ultrastructure of a cephalic sensory organ in larvae of the gastropod Phestilla sibogae (Aeolidacea, Nudibrachia).

The cephalic sensory organ is a superficial sensory receptor located between the velar lobes at the level of the shell aperture. Three cell types make up this sensory area: (1) six flask-shaped cells bearing numerous cilia: (2) adjacent supporting or accessory cells which have numerous, often branched, microvilli; and (3) vacuolated cells which occupy the center of the area. The flask-shaped cells appear to be the sensory units. These cells have a deep invaginated lumen, with ciliar arising from the cell surface in the lumen oriented either toward the base of the lumen or toward the epidermal surface these cilia, some of which extend slightly above the body surface, are presumed to be non-motile, as they lack (dynein?) arms on the axonemal A tubules and lack striated rootlets. The six flask cells are in intimate contact with the underlying cerebral ganglia and axons from each cell pass into ganglionic tissue. The supporting cells may be sensory, but no direct connection with the nervous system was seen. The function of the central vacuolated cells is not known. This cephalic organ may be a derivative of the original apical tuft of the trochophore stage.