32 ■ • CHORDATE ANATOMY 



external source. The embryo and young of the viviparous fish not only 

 receive maximum protection, but may obtain from the mother some food 

 in addition to the initial supply of yolk. In so-called "placental" sharks 

 the wall of the oviduct develops highly vascular folds or processes and 

 similar folds arise on the abdominal wall of the embryo. The two sets 

 of projecting structures, maternal and embryonic, become closely approxi- 

 mated, thus providing for diffusion of substances from the blood of one to 

 that of the other. 



Among amphibians there is, in general, better provision for protection 

 of eggs and young than in fishes. Nests and guarding of eggs are common. 

 Among frogs and toads occur various peculiar ways of caring for eggs 

 and young. The male of the European "obstetric" toad carries the long 

 strings of eggs wound about his body and legs until the tadpoles emerge. 

 In some cases eggs are carried in the mouth or vocal pouch of the male. 

 In the South American "marsupial" frog the eggs develop in a capacious 



Fig. 31. — Necturus larva of about 25 mm. length. (After Eycleshymer.) 



pouch formed in the skin on the back of the female. The eggs of the 

 toad, Pipa americana, develop in individual vesicles in the skin on the 

 back of the mother. Viviparity, affording a maximum of protection, 

 occurs in a few amphibians, including representatives of each of the three 

 orders, Urodela, Anura, and Gymnophiona. 



The amphibian egg, whether laid in the open or enclosed in some 

 protective way, develops rapidly into a highly characteristic larva, 

 the tadpole or "poUiwog" (Fig. 31) which, with its functional gills and 

 locomotor tail, as well as in many features of internal anatomy, is a dis- 

 tinctly fish-like animal and, if its environment is external water, it lives 

 the hfe of a fish. The larval period, ranging from a few weeks in some 

 salamanders to a year or more in some frogs, is devoted mainly to feeding 

 and growth. It terminates in a metamorphosis in the course of which the 

 animal acquires. the adult characteristics. The transformation is most 

 radical in frogs and toads; legs and lungs develop, tail and gills are 

 absorbed, gill clefts close, and other changes occur. In certain excep- 

 tional species of frog, especially large eggs are laid on land and develop to 

 adult form without passing through a tadpole stage. In the Urodela 

 the changes are less marked, the tail and sometimes also the gills being 

 retained. Adult Necturus, with its tail and functional gills, is sometimes 



