270 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



forms as Noctiluca among the Protozoa, by many 

 Medusae, by the Pennatulidae, by Beroe and Cestus ; 

 among the Annulata, it has been observed in the 

 earthworm, where it appears to have its seat in the 

 clitellum, and in various marine Polychseta ; in 

 Polynoe the light is of a greenish colour ; in Poly- 

 cirrus pale-blue ; among the Tunicata, Doliolum has 

 been observed to be phosphorescent ; and the com- 

 pound ascidian. Pyrosoma is, as its name implies, 

 remarkably so. As these animals float in great 

 companies, they have been spoken of as a " shoal of 

 miniature pillars of fire gleaming out of the dark sea, 

 with an ever-waning, ever-brightening, soft bluish 

 light " (Huxley), 



The physiology of phosphorescence is incompletely 

 known. Panceri observed in Pemiatula that the 

 activity was exhibited only by the eight longitudinal 

 bands of fatty substance placed on the outer wall of 

 the stomach ; and these bands are luminous after 

 removal from the body. They can be set in activity 

 by various stimuli, mechanical, chemical, and so on. 

 When exactly studied by electrical stimuli, there is 

 found to be a latent period of |ths of a second. The 

 fact that many deep-sea forms are coloured points to 

 the existence of light in great abysses of the ocean ; 

 this can only be due to phosphorescent animals, as we 

 cannot accept the supposition that sunlight can pene- 

 trate to any considerable depth. 



The observations of Aubert and Dubois on Pyro- 

 phorus, one of the well-known phosphorescent beetles 

 (Elateridse), have revealed the remarkable fact that 

 the most persistent of the rays of light are the green, 

 and that, with increasing brightness, the last rays to 

 appear are those that are least refractive, whereas, as 

 a rule, they are the first to be seen. This light has 

 been observed to have an action on sensitised paper. 



The colours of animals are due either to 



