THE ALLIES OF THE PERCH 



331 



and phosphorescent organs. Increase in size of mouth, jaws, 

 and stomach enables the fish to swallow an animal larger 

 than itself. Fig. 170 shows one of these species. 



Fishes of Past Ages. The earliest known remains of verte- 

 brates are those of fossil fishes. The oldest of the fossil 

 fishes were covered with a heavy shell, or armor, and in 

 appearance somewhat resembled arthropods. These fossil 



,— ;- T" , X^-' ,T W^ ? *^*^- v ^^ v y^t 



Fig. 171. Two examples of fossil fishes 



fishes are found in rocks of that period of the earth's his- 

 tory which is known as the Age of Invertebrates. Geologists 

 very commonly call the following age the Age of Fishes, for 

 then many different kinds of fishes made their appearance. 

 Some of these were not very different from present-day 

 fishes (Fig. 171), while others represent groups unlike any 

 that exist today. Paleontologists tell us that fishes first ap- 

 peared on the earth about twenty-five million years ago. At 

 that time they were the highest animals on the face of the 

 earth. But we should hardly recognize as fishes some of 

 the species of that day. 



