. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Bioluminescence 661 duced intermittently, and commonly only in response to external stimuli. There are numerous exceptions to this, however. Many truly photogenic cells of animal origin exhibit a continuous glow. On the other hand, lumi- nescence of bacterial origin can be made to give the semblance of intermit- tency as, for example, in the fish Photohlepharon, where the symbiotic lumi- nous bacteria are in a light-producing pouch with a lid capable of intermit- tently screening the light. Occurrence of Biolumi
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Bioluminescence 661 duced intermittently, and commonly only in response to external stimuli. There are numerous exceptions to this, however. Many truly photogenic cells of animal origin exhibit a continuous glow. On the other hand, lumi- nescence of bacterial origin can be made to give the semblance of intermit- tency as, for example, in the fish Photohlepharon, where the symbiotic lumi- nous bacteria are in a light-producing pouch with a lid capable of intermit- tently screening the light. Occurrence of Bioluminescence Among Animals. Among the Protozoa numerous luminescent species are observed. These are marine radiolarians and dinoflagellates. The best known example of the latter group is Noctiluca. The light-producing granules are located throughout the organism but par- ticularlv in the peripheral regions.^** The organisms glow briefly in response to stimulation; the glow originates in the region of the oral groove and spreads as a wave over the body.'''^. Fig. 250 Chaetopterus. A, Dorsal view (after Trojan). B, A luminescent individual in darkness (after Panceri). From Harvey.^^ Numerous coelenterates are known to be luminous. These include a num- ber of hydroid polyps, iellyfishes, siphonophores, and sea-pens. In this group the region luminescing is usually the whole surface of the organism, but luminescence may be restricted to certain spots. Luminescence occurs only in response to stimuli. The natural stimulus is probably mechanical. The region immediately stimulated is first to respond, and this response is followed by a wave of luminescence proceeding out from that point.^-' '^^ In the jellyfish, Pelagia, the extent of spread of the luminescence is a direct func- tion of the strength of the stimulus.'^^ Light production in coelenterates ap- pears to be largely of the extracellular tvpe, since luminous mucus can usu- allv be readilv rubbed from the surface of the ^
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