Ocelli of hydromedusae.

Abstract

The very simple ocelli of Leuckartiara octona are formed of a sheet of mixed ectodermal receptor and pigment cells, whereas in Bougainvillia the cell sheet is invaginated to form an ocellar cup. The ocellar cup of Tiaropsis is formed of endodermal pigment cells. The receptor cell bodies of Tiaropsis lie outside the cup, their distal processes filling the cup. Each receptor cell process bears a single cilium at its distal end except in Bougainvillia where the number of cilia may vary from one to three. Lateral microvilli are missing from the ciliary membrane of Leuckartiara, poorly developed in Tiaropsis and well developed in Bougainvillia. The proximal part of the receptor cell forms an axon which enters into the subumbrellar nerve ring in Tiaropsis and exumbrellar nerve ring in the other two. The pigment cells lack distal processes in Tiaropsis, in Leuckartiara they bear microvilli and in Bougainvillia they 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. Bougainvillia medusae 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 discussed.