Davenport, D. and Nicol, J.A.C.
Luminescence in Hydromedusae.  Proceedings of the Royal Society of London Series B-Biological Sciences 144: 399-411 (1955).

The luminescent responses of Hydromedusae have been investigated by means of photoelectric recording. Animals examined included Aequorea, Halistaura, Phialidium and Stomotoca; the first proved most suitable for experimentation. Light appears in a ring around the periphery of the umbrella. In Aequorea the luminous points correspond to yellow-green fluorescent masses in the marginal canal. The photogenic tissue in histological preparations appears as oval masses of eosinophilic cells bulging into the cavity of the marginal canal. Luminescence is intracellular. With carefully localized mechanical and electrical stimulation, light appears in one or a few restricted loci; there is no spread of luminescence around the margin. The luminescent response shows facilitation, summation at higher frequencies, and fatigue under continued stimulation. Each flash in Aequorea lasts 0.3 to 1.5 s; maximal intensity is reached in 0.1 s (14 to 16 degrees C). It is concluded that the responses observed involve either direct excitation of the photocytes, or stimulation through local nervous pathways.