VISUAL PROBLEMS 231 



and its viscera are often blown out of the body-cavity 

 by the expansion of the air-bladder. 



The immediate results of the absence of sunlight 

 are not so easy to trace, because there is no doubt 

 that the depths of the sea are more or less lit up 

 by the phosphorescence of their own inhabitants. If 

 the bottom of the sea were quite dark, we should 

 expect that the fishes living there would, like those 

 of underground caverns, be blind. But, unfortunately, 

 we can seldom be sure, of any given fish brought up 

 in the dredge, either that it actually came from the 

 bottom, or that, if it did, it really passed the whole 

 of its life there and never at any season removed into 

 regions that are in some way or other illuminated : 

 and so long as we are uncertain on these points we 

 must not be surprised to find that the majority of 

 deep-sea fishes have eyes, and that many of them 

 have eyes of extraordinary size. Where, however, we 

 bring up, from depths to which no sunlight can 

 penetrate, fishes that, either from their habits or from 

 their structure, we know 7nust have lived on the 

 bottom and nowhere else, we often do find, especially 

 if such species have no means of producing their own 

 light, that they are either quite or nearly blind. 

 Examples of such species will be described further on. 



One result of the absence of daylight is that many 

 of the deep-sea fishes have in a high degree the 

 power of manufacturing their own light. In most of 



