STRUCTURE OF LUMINOUS ORGANS 71 



thys, which can only be stimulated to luminesce during the 

 spawning season in spring and early summer. 



Some animals possess a periodicity of luminescence. 

 They only luminesce at night and fail to respond to stimu- 

 lation or are difficult to stimulate during the day. Bright 

 light has an inhibiting effect. Perhaps correlated with 

 this is the fact that most luminous forms are strongly 

 negatively heliotropic. Fireflies lie hidden in the day, to 

 appear about dusk and the ostracod crustacean, Cypri- 

 dina, is difficult to obtain on moonlight nights. 



The Ctenophores were the first forms in which the 

 inhibiting effect of light was noticed. This was described 

 by Allman (1862) and has been confirmed by a numbet of 

 observers, especially Peters (1905). Massart found that 

 Noctiluca was difficult to stimulate during the day and 

 Ceratium, according to both Zacharias (1905) and Moore 

 (1908), only luminesces at night, or if kept in darkness, 

 for some little time. Crozier * finds a persistent day-night 

 rhythm of light production when Ptychodera, a balano- 

 glossid, is maintained for eight days in continued dark- 

 ness. The animal is difficult to stimulate during the 

 period which corresponds to day and luminesces brill- 

 iantly and at the slightest touch during the period which 

 corresponds to night. 



On the other hand, a great many forms are able to 

 luminesce quite independently of previous illumination. 

 According to Crozier * Chcetopterus luminescence is not 

 affected by an exposure to 3000 metre-candles for 

 six hours. 



In the case of aninials with extracellular luminescence 

 we may speak of luminous secretions and true luminous 



* Private eommunication. 

 6 



