106 How Animals Changed 



in their investigations in the tropical swamps of the Gran Chaco 

 found four types of air-breathing organs in fishes: (1) lung-like air 

 bladder, (2) large gill chamber without special structural adapta- 

 tions, (3) vascular intestine with contained air bubbles, and (4) 

 vascular stomach with contained air bubbles. They give a list of all 

 air-breathing fishes in the world, with their types of adaptations. 



Amphibians furnish an interesting series of types ranging from 

 those which are aquatic in all stages in their life cycles (Siren, Nec- 

 turus) to those which are completely terrestrial (Plethodon) . Many 

 begin their lives in water as tadpoles and metamorphose later into 

 land animals. The American newt, Triturus viridescens Rafinesque, 

 may remain aquatic throughout its life (Noble, 1929) or spend two 

 or three years on land, and then return to the water to breed. "The 

 subjection of the incompletely metamorphosed newts to terrestrial 

 conditions causes a reduction of the gill stubs. . . . Siren is a form 

 which has ceased to differentiate most of its structures beyond the 

 stage characteristic of the early larva" (Noble, 1929) . Morgan dc 

 Sondheim (1932) found that keeping gilled newts in a dry environ- 

 ment caused no reduction of gills, but the transplantation of por- 

 tions of anterior pituitary lobe into the bodies of animals induced 

 both males and females to become sexually mature though still 

 bearing gills. 



A frog tadpole is in a state somewhat comparable to an adult 

 dipnoan. According to Willem (1920), a frog passes through five 

 stages in regard to its respiration: (1) tadpole with internal gills 

 like a fish, (2) tadpole with perforate nostrils and buccal respira- 

 tory movements, (3) young frog depending on cutaneous and buc- 

 copharyngeal respiration, (4) terrestrial young frog still dependent 

 upon cutaneous and buccopharyngeal respiration, and (5) a frog 

 dependent largely on lungs but still using skin when necessary. Cer- 

 tain salamanders that live in brooks and on land have no lungs and 

 depend on buccopharyngeal and cutaneous breathing. The cuta- 

 neous respiration of a frog is not under the control of the nervous 



