CHAPTER XVII 

 LAMPREYS AND FISHES 



Fishes belong in the water and nowhere else ; there they find 

 their food and produce their young. Frogs shed their eggs 

 in the water but range over the land for something to eat; 

 pond turtles forage through the shallow waters but deposit 

 their eggs on land. Water birds go to it only for food and 

 safety but never lay their eggs in it. No mammal, not even 

 a whale, can stay under water indefinitely, and the life of 

 aquatic mammals like muskrats and beavers is one of constant 

 come and go from water to land. But fishes are ever-present 

 in water society — the only group of vertebrates which can 

 survive a constant water environment. 



Fishes have taken possession of the streams and lakes as 

 well as the sea. They are surface skimmers and bottom feed- 

 ers, dwellers in rapids and in still coves; they are big and little, 

 long and slender or short and thick, drab gray or silver and 

 gold colored, metallic blue and green. 



Structure of fishes. — All are vertebrates. They breathe 

 by means of gills; they all have fins, and they are usually 

 covered with scales. Most of them lay eggs which are ferti- 

 lized in the open water as soon as they are laid, but some 

 give birth to living young. The eggs of most marine fishes 

 float upon the surface, but those of fresh water fishes lie loosely 

 on the bottom or are stuck fast to stones. All fish eggs are 

 well stocked with yolk, and the young fish carry a yolk-sac 

 about with them for several days after they hatch and are 

 thus well provided with food wherever they go. 



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