THE CHARACTERS OF THE DEEP-SEA FAUNA 71 



fathoms the eyes of some fish become considerably 

 reduced, but those of others become still more en- 

 larged. In the greatest depths of the ocean in fact 

 it seems very probable that nearly all the fish are 

 characterised by either very large eyes or very small 

 ones. 



We cannot expect to learn very much at present 

 from the study of the eyes of deep-sea mollusca. 

 The Cephalopoda form the only class of this Phylum 

 whose genera invariably possess large and well- 

 developed eyes, and there does not seem to be any 

 very marked increase or decrease in the size of the 

 eyes of the few deep-sea cuttlefish that are known 

 to us. 



The eye of Xtutti'lnx is certainly remarkably 

 interesting, but as this genus is the only represen- 

 tative of its order, and is known at times to float 

 upon the surface of the ocean, it would certainly be 

 erroneous to attribute the peculiarity of the structure 

 of its eye to its 'temporary' deep-sea habits. We are 

 still ignorant of the usual habitat of the remarkable 

 genus Spirula, notwithstanding the fact that many 

 of the tropical beaches are very largely composed of 

 its empty shells. Whether it is a deep-sea dweller or 

 not, we know nothing at present of the character of 



