THE AQUATIC VERTEBRATES 1029 



eggs may be delayed much beyond the normal time and will then 

 occur at once with the first rain. The young still require a pond 

 for their growth from hatching to the metamorphosis. Toads and 

 frogs have evidently become adapted to range on land without 

 losing their ancestral habit of making their home in water. Whether 

 their webbed toes and swimming legs are in their original condi- 

 tion, or whether they are readaptations to water may be left in 

 abeyance. 



The batrachians play an important part in the economy of small 

 pools, a less important one in small streams, and are a negligible 

 quantity in waters of any size. To the rule that their abundance 

 is in inverse proportion to the size of the body of water, the perenni- 

 branchs form the only exception. In early spring nearly every 

 puddle contains hundreds or thousands of toad eggs and larvae. 

 The tadpoles act as scavengers for a short time and then pass out 

 of the life of the puddle. Every pond of greater permanence 

 serves the frogs as the puddles and ponds do the toad. Frog tad- 

 poles are scavengers and mud eaters, with elongate, alimentary 

 canal. They remain in the water much longer than young toads 

 and when they become adult may pass out of the life of the puddle 

 or pond as completely as the adult toad, or may remain more or 

 less closely identified with the birthplace. When the adult frogs 

 remain about the water, they bear a different relation to the 

 aquatic life from the young. The alimentary canal has become 

 shortened and the frog is an eater of live food, insects, and fishes. 

 In its turn the frog serves as food for fishes, snakes, and birds. 



FISHES 



The chief and perennial vertebrate elements of the aquatic fauna 

 are the fishes. They, with a few batrachians and possibly a turtle, 

 are the only members of the fauna that have both their home and 

 their range in the water. They alone of the vertebrates are so 

 adjusted to an aquatic existence that they could be hermetically 

 Bealed in a balanced aquarium. 



There are fishes, big and little, thick and thin, long and short, 

 deep, and of little elevation, sharp-nosed and blunt-snouted, tooth- 



