572 Comparative Animal Physiology 



spread ot luminescence over a stimulated animal proceeds in all directions 

 at a rate characteristic of nerve-net transmission.^*^ In the ctenophore, Mnemi- 

 opsis, the tactile receptors involved in reflex light production lie along the 

 rows of ciliated combs.^^ 



Most studies of the control of luminescence ha\'e concerned fireflies. 

 Among other stimuli, visual cues normally excite a responsi\'e flash in these 

 insects. The photogenic organ is innervated, but it is not definitely estab- 

 lished whether the nerve supply goes to the photogenic cells proper or to 

 the tracheal endings within the light organ. In the latter case, the nerves 

 might be supposed to innervate a complex valve mechanism located in the 

 tracheal end-cell. 



Two general types of theories for the control of flashing in the firefly 

 have been advanced. The first type presumes that the flash depends on the 

 rapid and transitory admission of oxygen to the photogenic cells. This is 

 considered to be brought about either by direct ner\'ous control of \'alves in 

 the tracheal end-cells or by means of a stimulated increase in the metabolites 

 in the photogenic tissue and a consequent osmotic withdrawal of water from 

 the terminal portion of the tracheal tubules with the result that oxygen 

 would be brought directly to the glandular elements. The presence of oxy- 

 gen would then permit an oxidation of the metabolites, again reducing the 

 osmotic pressure of the cell contents, and permit restoration of water to the 

 tubules and exclusion of the oxygen. In support of an oxygen-control mech- 

 anism is the fact that microscopic observations of a luminescing gland 

 shows the brightest light to come from the immediate vicinity of the ends of 

 the tracheal tubules. An end-cell valve mechanism is not essential, how- 

 ever, since some insects, especially larvae of fireflies, which show at least 

 some degree of intermittency in light production, even though not true 

 flashing, do not have diflr'erentiated tracheal end-cells. 



The second general type of mechanism postulated is found in numerous 

 other animals which also flash, although they possess no tracheal mechanism. 

 This is a direct nervous excitation of the light-producing cells through photo- 

 genic nerves. 



In the fishes, Porichthys and Echiostoma, there is some evidence that the 

 normal reflex pathways of control of the system of photogenic organs in- 

 cludes the secretion and action of the hormone adrenin. 



There have been a number of observations indicating that bright light 

 inhibits the production of light by animals,'^' ^^' ^- although in most cases 

 we know little or nothing of the mechanism of this inhibition. It may result, 

 as some evidence indicates for ctenophores, from a direct destruction of 

 photogenic material within the light-producing cells. In these animals ex- 

 tracts made from specimens exposed to sunlight show practically no capacity 

 to luminesce. Furthermore, after exposure to bright light these organisms 

 have to be kept in darkness often for as long as 45 minutes before they will 

 again luminesce. 



There have, on the other hand, been numerous reported observations of 

 the existence of a diurnal rhythmicity in the capacity of certain species of 

 animals to luminesce. Such a rhythm has been described to persist for sev- 

 eral days in constant darkness in the firefly Photinus,'^ in the balanoglossid, 

 Ptychodera,^- and questionably in the jellyfish, Pelagia.^^ Such behavior 



