328 ZOOLOGY 



The first animals to come to the land were probably the 

 earthworms, many of whose aquatic allies live in ponds that 

 are apt to dry up during the summer. The acjuatic worms are 

 preserved from annihilation only Ijy burying themselves in 

 the muddy, damp bottom. One of the next groups to give 

 rise to terrestrial forms was that of the Arthropods, some of the 

 representatives of which still retain in their bodies evidences 

 of their aquatic ancestry. Thus in the terrestrial Peripatus 



Fig. .SIO. — Peripatus, the air-breathing wormlike animal that bridges the 

 gap between Myriapods and worms. 



(Fig. 310) the air-breathing organs have not yet developed, and 

 the appendages are placed laterally instead of ventrally. From 

 some such primitive form the plant-eating insects arose, and 

 on account of the almost limitless supply of vegetation devel- 

 oped mightily. Then came the carnivorous forms, like the 

 spiders and their allies, to prey on their herbivorous forerun- 

 ners. 



Among the vertebrates the primitive forms lived in the 

 water, but in the group Amphibia, which includes the newts, 

 salamanders, frogs, and toads, there are forms that show the 

 transition from aquatic to terrestrial life. Most of them 

 begin life as aquatic animals and end it as terrestrial. 



The Amphibia comprise about a thousand species which are 

 distributed in three great groups. Those forms that retain 

 their tails permanently, even when they come to land, are 

 known as Urorle'la. They are confined in their distribution 

 to the Northern Hemisphere, excepting two or three species 



