Ocelli of hydromedusae

摘要

The very simple ocelli ofLeuckartiara octonaare formed of a sheet of mixed ectodermal receptor and pigment cells, whereas inBougainvilliathe cell sheet is invaginated to form an ocellar cup. The ocellar cup ofTiaropsisis formed of endodermal pigment cells. The receptor cell bodies ofTiaropsislie outside the cup, their distal processes filling the cup. Each receptor cell process bears a single cilium at its distal end except inBougainvilliawhere the number of cilia may vary from one to three. Lateral microvilli are missing from the ciliary membrane ofLeuckartiara, poorly developed inTiaropsisand well developed inBougainvillia. The proximal part of the receptor cell forms an axon which enters into the subumbrellar nerve ring inTiaropsisand exumbrellar nerve ring in the other two. The pigment cells lack distal processes inTiaropsis, inLeuckartiarathey bear microvilli and inBougainvilliathey form a strand of 1μ diameter which passes through the ciliary region and then divides into a number of tubules to fill the distal region of the ocellar cup.Bougainvilliamedusae showed a swimming “off response” to light of 446–625 nm wavelength but no such response was observed when the ocelli are removed. The evolution of ocelli in hydromeduase is dis