Staocysts of hydromedusae.

The statocysts of Leptomedusae are formed as a depression in the velum. They are lined on the inside towards the distal part of the velum by thin epithelium and towards the proximal part by ciliated sensory cells. Lithocytes are present in the centre. The concretion contains calcium sulphate and in some cases, calcium phosphate is also present in addition to some membranous material. The statocysts of Narcomedusae arise from the exumbrellar nerve ring as free sensory clubs. They have a proximal basal cushion of sensory cells from the centre of which arises a sensory club (Aegina) or a sensory papilla carrying a sensory club (Solmissus). The sensory club has an axial strand of endodermal cells covered by ciliated sensory cells. Some of the endodermal cells have a concretion. While the statocysts of Leptomedusae are totally ectodermal, those of Narcomedusae are ecto-endodermal in origin. The sensory cilia of Leptomedusae, especially those present on the sensory cells adjacent to the lithocyte, run close and parallel to the lithocyte membrane. In Narcomedusae the sensory cilia of the basal cusion and sensory papilla are tall and strong. Ciliary rootlets are missing in the sensory cilia of Leptomedusae and in the sensory club of Narcomedusae but they are strongly developed in the cilia of basal cusion and sensory papilla. The cilia have 9+2 filament content. A ring of stereocilia surrounds the kinocilium of the sensory club cells. Mechanism of statocyst function is discussed.