128 THE FAUNA OF THE DEEP SEA 



species living in deep water ranging from 400 to 1,975 

 fathoms, we cannot fail to see that the latter are 

 undergoing a process of degeneration ; the retinulas 

 and pigment being absent, and nothing left of the 

 complicated structure of the Isopod eye but the 

 remnants of the crystalline cones and corneal facets 

 (see figs. 4 and 5, p. 74). 



Taking the genus Serolis alone, it has been 

 said ' that in all the shallow-water forms the eye 

 is relatively small but very conspicuous from the 

 abundant deposition of pigment ; in all the deep-sea 

 forms, with the exception of S. gracilis, where the eye 

 seems to be disappearing, it is relatively larger but 

 not so conspicuous, owing to the fact that little or no 

 pigment is present.' 



In many groups of animals it has been shown 

 that some of the deep-sea species are relatively much 

 larger in size than the shallow-water species, and 

 that others, more rarely, are much smaller, the 

 abysmal fauna reminding us in this respect of the 

 characters of the alpine flora. 



The Tsopoda show many examples of this large- 

 ness in size, thus Bathynomus c/iganteus, dredged by 

 Professor Agassiz off the Tortugas at a depth of over 

 900 fathoms, reaches the enormous size, for an Isopod, 



