36 ANIMAL COLOKATION. 



(among crinoids, sea-urchins, etc.) ; blue, however, is not absent; 

 Professor Agassiz remarks that " no blue animals have been 

 noticed among the deep-sea types," excepting only in the case 

 of a small sponge not uncommon in the neighbourhood of the 

 hundred-fathom line in the Gulf of Mexico. I am not aware 

 if Sir Wyville Thomson's description of a star-fish, Porcellana- 

 ster, as being of a blue colour was at all exaggerated ; but the 

 late Dr. Von Willemoes-Suhm described a large species of the 

 Isopod genus Serolis as " of a fine blue colour," which colour is 

 partially visible even in the preserved specimens of that species. 



So far, therefore, there appears to be nothing unreasonable in 

 this theory, which depends upon two apparently undoubted 

 facts : (1) light due to phosphorescence, (2) the general 

 presence of eyes. 



But while eyes are apparently often present among deep-sea 

 animals, they are not unfrequently entirely absent ; this fact 

 would not fit in with the theory, if the eyeless forms belonged to 

 genera or families of which the shallow-water species invariably 

 have eyes. As to the blind gastropods and fishes of the deep 

 sea, many of them live in mud, where eyes would be useless to 

 them in any case. 



Among the Crustacea, where the eyes are more conveniently 

 studied than in many other groups, eyes may be present or 

 absent ; but this does not necessarily offer an unsurmountable 

 obstacle to the theory of abyssal light, because some shallow- 

 water species may also be without eyes ; among the Isopoda,* 

 for example, the genus Pleurogonium, confined to shallow 

 water, is totally blind ; so are the genera Munnopsis, Eurycope, 

 IscJmosoma, Typhlotanais and Cryptocope, whether they occur 

 in deep or shallow water. 



* See my Report on the Isopoda collected by the Challenger (" Zool. Chall. 

 Exped.," Pts. xxxiii. and xlviii.) 



