Fine structure studies of the ocelli of Polyorchis penicillatus (Hydrozoa, Anthomedusae) and their connection with the nerve ring

Summary

The fine structure of the small compact ocelli (50–100 μm in diameter) of Polyorchis penicillatus is described. The ocellar cup is formed of pigment cells and receptor cells. The pigment cells occur in approximately a 2:1 ratio to the receptor cells. Each pigment cell has a process that may pass through the presumed photosensory region. Pigment cells are connected to adjacent receptor cell processes by septate junctions. The sensory cells are bipolar with the apical part forming the receptor process and the basal part forming an axon 8–15 μm long and 1–2 μm in diameter. Each receptor cell axon forms a synapse with a single second order neuron but the sensory cells are also connected to the second order neurons postsynaptically. There are also synapses between adjacent second order neurons. The second order neurons lie outside the ocellar cup, next to the tentacular mesogloea. Each second order neuron forms an axon of about 1 μm thickness. The axons on each side group together to form an optic nerve having 30–40 axons that travel around the tentacle base on either side and enter the outer nerve ring independently.