28 NATURAL HISTORY OF VERTEBRATE ANIMALS 



the flying fish, and a few species that remain on the moist sea- 

 weed between tides or are capable of short journeys on land. 

 These, however, are quite exceptional. The whole organization 

 of fishes shows their fundamental adaptation to an aquatic life. 

 The Amphibia (Fig. 1 B) are animals whose early stages, the tad- 

 poles, are fish-hke in structure and thoroughly adapted for Hfe in 

 the open water, save in a few species where some form of brooding 

 the young occurs (Fig. 14) ; and even in such instances the embryo 

 is surrounded by a fluid during its development. In its final 

 stages the animal may five permanently in the water; but most 

 forms become " amphibious," breathing air by means of lungs, 

 although the skin is also used in respiration (c/. p. 91). Hence, 



the Amphibia, as a class, are 

 confined to the vicinity of 

 water, as with the frogs, or to 

 regions of moist atmosphere, 

 as with the toads. 



The Reptiha, on the other 



^ ,. , , , . , hand, possess fully developed 



Fig. 14. — A tree frog, i/wia ooeWii, which , , , • ,i . . . 



,, .'•"...,' , lungs and a skm that protects 



carries the eggr; m an incipient brood- ° ^ 



pouch on its back until the hatching their bodies from excessive 



stage, which is probably as late as the evaporation, and are true land 



time of metamorphosis. animals. The developing em- 



(From The Cambridge Natural History, bryO is encloSCd in an egg 

 copyright, 1920, by Macmillan & Co., Ltd., i i, i_ • i • i j.u IM 



reprinted by permission.) shcU whlch IS leathery hkc 



that of a turtle, not brittle like 

 that of a bird (Fig. 220, p. 423). Since these egg shells resist 

 evaporation sufficiently to allow development in a moist atmos- 

 phere, the reptiles find it unnecessary to lay their eggs in water 

 as do the Amphibia. They are thus "emancipated" from the 

 water. There are a few cases among the reptiles, for instance, in 

 some of the snakes, where the young are born "alive" (c/. p. 423). 

 Birds are similar to reptiles in their manner of development, save 

 for the difference in the egg shell and for the care given the young 

 after hatching. 



Of all vertebrates, the mammals are the most highly special- 

 ized for terrestrial life. In most species, the early stages of 

 development are passed within the body of the female parent 

 but the most primitive mammals (Fig. 15) lay eggs that 

 resemble those of birds and reptiles (r/. Fig. 224, p. 428). In some 



