Putative extraocellar photoreceptors in the outer nerve ring of Polyorchis penicillatus

Putative sensory cells have been described in the outer nerve ring of the hydrozoan jellyfish Polyorchis penicillatus that resemble ocellar photoreceptors characteristic of this animal group. Each receptor cell bears an apical structure consisting of verticillate lamellae representing evaginations of ciliary membranes (9 + 2 axonemes). Verticils are grouped in clusters of three or more and are distributed throughout the outer nerve ring. Individual lamellae of each verticil are up to 0.2 μM wide and are frequently reflexly folded to form a tightly packed whorl. Multiple axonemes contribute to each verticil. Each receptor cell body contributes a single axon (up to 1.75 μm in diameter) to the neuronal tracts of the outer nerve ring. The structural similarity between the described cells and hydrozoan photoreceptors, and the close agreement between the location of these cells and the localized photoresponse in Polyorchis and related medusae suggest that the putative receptor cells function as photoreceptors.