THE CHAEACTEES OF THE DEEP-SEA FAUNA 73 



firmly fixed, greater in length, and take the place 

 of the rostrum which disappears. In some forms 

 -such as Thaumastocheles zaleuca and Willemoesia 

 the eye-stalks themselves have completely dis- 

 appeared. 



In the deep-sea Isopoda some forms lose their eyes 

 entirely, but Bathynomus giganteus possesses a pair 

 of enormous eyes, each provided with 4,000 facets. 



To illustrate the distribution of eyes in this group, 

 we may take as an example the genus Serolis. All 

 the species of this genus are provided with eyes 

 except Serolis antarctica a species that extends from 

 600 to 1,600 fathoms. 



The eyes of all the deep-sea species are relatively 

 larger than those of the shallow- water ones, except 

 Serolis gracilis, whose eyes seem to be disappearing. 



But these large eyes of the deep-sea species of 

 Serolis are not capable of any greater perceptive 

 power. In fact, the evidence of degeneration they 

 show, both in minute structure and in the diminution 

 of pigment, proves that they can be of very little use 

 to these animals for perception (see Figs. 4 and 5). 



This increase in size, accompanied by degeneration 

 of structure, is just what we should expect to find in 

 the eyes of deep-sea animals, and it is difficult to 



