PROTBCTIYK COLOEATION. 123 



I have already referred on another i)age to the gor- 

 geous and diverse coloration of the littoral and deep-sea 

 nudibranchs. This group of mollnscs has one representative 

 among the pelagic fauna; the animal was named by its dis- 

 coverer, Forster, the comiianion of Captain (\oQk,Glaucus; it is 

 of a dark blue colour. 1 have never myself had the opportunity 

 of seeing Glaiicus in a living condition, but I have seen Velella 

 on the south coast of Spain ; in the shallow water it was a 

 tiny but extremely conspicuous object; perhaps this is not the 

 case when it is floating in waters far removed from the shore. 



Perhaps the most remarkable case of apparent adaptation 

 in accordance with a pelagic life is afforded by the transparent 

 fish Leptoccphalas ; this long and narrow, perfectly trans- 

 lucent fish received the name Lcptocephalus in the belief 

 that it was an adult form ; it is now known to be merely the 

 young of the Conger. It is not only perfectly colourless as 

 regards the skin, but the blood, which should be red as in all 

 other vertebrates, is also colourless ; this instance, iipon which 

 stress is generally laid by those who are convinced of the 

 eifects of natnral selection in rendering pelagic organisms 

 transj)arent, certainly offers to that view strong support. This 

 is not the place to attempt any comj^rehensive account of 

 the characteristics of pelagic life ; those who desire farther 

 information upon the topic are referred to the books mentioned 

 in the footnote to page 121. Enough has been said in illustra- 

 tion of the fact that pelagic organisms, if not perfectly pellucid, 

 are coloured in such a way as to render them inconspicuous to 

 our eyes at least. 



The members of the surface fauna — the more highly organ- 

 ised members, at any rate — possess, however, every facility for 

 seeing each other. Their eyes are, as a rule, very well developed. 

 The amphi])()d crustacean P/trommn (fig. s) is " not satisfied," 



