Luminescence is well known in three groups of coelenterates, viz. Hydrozoa, Scyphozoa and Alcyonaria (literature reviewed by Harvey, 1952). Of these Pelagia, Pennatula and Renilla have been most intensively investigated. Observations indicate that in all these forms, light emission is under nervous control. Mechanical and electrical stimulation gives rise to a wave of light which passes over the surface of the animal (the disk in medusae, or the surface of the colony in hydroids and alcyonarians), at a rate which is consonant with nervous transmission as deduced from the spread of excitation concerned with muscular contraction. In his pioneering studies of the physiology of luminescence, Panceri (1872 a, b) found that localized stimulation of Pennatula excites luminous waves which spread up and down the colony, according to the position of excitation. Simultaneous excitation of the two extremities of an animal gives rise to two converging waves which cancel out on meeting. Parker’s (1920b) detailed study of Renilla, which will be referred to later, showed that luminescence is controlled by a nerve net, and that cuts made in any direction through the rachis still permit excitation to be transmitted so long as the stimulated region is not completely isolated, i.e. the nerve net is non-polarized.

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Personal observation of E. Noble and D. Davenport.

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