i] THE CATEGORIES OF LIFE 21 



for instance, does not differ greatly in structure from 

 a whiting, though the one is a pelagic, surface-living 

 animal and the other is one which nearly always lives 

 at the sea-bottom, and both have swim bladders. 

 But in the case of the oceanic fishes the structure 

 may be strikingly different according to the depth of 

 water which the animal usually inhabits. A flying 

 fish 'differs very notably from any of the abyssal 

 forms which we have already considered ; and those 

 species which live at intermediate depths are also 

 very remarkable in form. They are, as a rule, small, 

 very delicate in structure, with semi-transparent 

 bodies, often compressed from side to side. It is 

 quite easy to read ordinary print through the body 

 of the peculiar Leptocephalus larva of the eel. Many 

 of them are tinted in shades of violet or silver-blue, 

 and have a peculiar shimmering appearance. Often 

 their eyes telescope into sockets. As a rule these 

 are the characters of the pelagic fishes found in the 

 oceans, in the intermediate layers down to a depth 

 of about 200 fathoms. Below that level the colour 

 is usually black or dark red, and the form of the fish 

 is that which we have already recognised as charac- 

 teristic of the abyssal category. 



All the animals and plants which we have dealt 

 with so far are such as can easily be seen with the 

 naked eye, or which can be captured by means of the 

 nets and lines of the fisherman. But everywhere the 



