GENERAL ZOOLOGY 



colors are sometimes developed; by modifications of the 

 eyes, either very large in proportion to the size of the fish, 

 or entirely absent; by the development of tactile and phos- 

 phorescent organs; and by hypertrophy (increase of size) of 

 mouth, jaws, and stomach, which enables the fish to swallow 

 an animal larger than itself. Fig. 161 shows one of these 

 species. 



Geological Development of Fishes. We left the geological 

 history of the development of animal life on the earth at the 



FIG. 162. Devonian Fishes 



end of the Age of Invertebrates. A glance at the map on 

 page 295 will serve to recall the appearance of our continent 

 at that time. In North America the Age of Invertebrates 

 passed slowly and quietly into the Devonian Period, or Age 

 of Fishes. Though numerous kinds of invertebrates already 

 in existence rose into prominence, or passed away to give 

 place to new forms, the chief interest centers about the 

 fishes. Geology gives us no hint concerning the remote an- 

 cffctry of the class. The earliest fossil fishes, which belong 



