Fine structure of the sensory receptors of Aglantha digitale (Hydromedusae: Trachylina).

Fine-structure studies of the sensory receptors of Aglantha digitale have been done by transmission- and scanning-electron microscopy. Aglantha has 8 club-shaped statocysts and three types of ciliated sensory receptors: ciliated comb pads, ciliated sensory cells with microvilli, and ciliated sensory cells without microvilli. The epithelium covering the statocyst has both ciliated sensory cells and epitheliomuscular cells. The ciliated comb pads are formed by a group of ciliated sensory cells, each having an apical kinocilium surrounded by a ring of microvilli. These microvilli show an increase in length towards the velum. The axons from the comb pad cells enter the inner nerve ring. The structure, function and the evolution of the statocysts and the possible role of the ciliated comb pads and other sensory receptors are discussed.