ANIMAL LIFE 171 



small light-organs, while one species (C. 

 signata) lives in much shallower water, is grey 

 in colour, and has large light-organs. The 

 surface forms of the scopelid genus Myctophum 

 have large light-organs, while the deep-water 

 forms of the same genus have very small 

 light-organs. During the " Michael Sars " 

 Expedition in the North Atlantic the fishes 

 taken in depths beyond 400 fathoms had 

 usually either no light-organs or very small 

 ones, while those taken between 100 and 300 

 fathoms had large light-organs. 



Development of Eyes. — Evidently the size 

 of the eye is correlated with phosphorescent 

 light and the penetration of sunlight, for 

 among pelagic fishes a great change in the size 

 of the eye is noticeable in those living near 

 the lower limit of the photic zone as com- 

 pared with those living in the lesser depths. 

 Thus in many of the fishes taken from depths 

 of 100 to 300 fathoms the diameter of the eye 

 is about one-half to one-quarter of the length 

 of the head, while in those taken beyond 300 

 fathoms the eye may be only one-twelfth or 

 one-fifteenth of the head, or it may be absent 

 altogether. It is interesting to note that the 

 only blind squid known was taken at a depth 

 of 800 fathoms. Among the Crustacea a 

 decrease in the size of the eye with increase 

 of depth has also been noticed : in many 



