374 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



the other, but by substitution of the one for the other. 

 Some steps we have already seen in the higher fishes, 

 the others we find in the lower amphibians. In Ganoids 

 we saw a little supplementing of gill breathing by the 

 taking of air into a vascular air bladder. This is the 

 beginning of lung breathing. In Dipnoi vie. saw an equal 

 gill breathing and lung breathing. Now, in the lowest 

 amphibians, such as the siren and others, we find but 

 little advance on the Dipnoi. These breathe both by 



gills and by lungs 

 all their lives. 

 They do not lose 

 their gills. They 

 are called perenni- 

 branchs. The nec- 

 turus and the siren 

 are examples of 

 these (Fig. 257). 

 In the next higher 

 amphibians the gill 

 breathing is confined to their early life. They drop 

 their gills in maturity. These are called caducibranchs. 

 The salamanders and water dogs are of this kind. In 

 the highest amphibians, such as frogs and toads, the 

 changes are greater. These lose their gills somewhat 

 earlier, and also lose their tails, and are therefore called 

 anura, or tailless. Therefore amphibians may be classi- 

 fied thus : 



Anura — tailless, such as frogs and toads. 



ffmfc/a-tailed, \ Caducibranchs. 

 ( Perennibranchs. 



Here there is one great distinction between am- 

 phibians and reptiles. Amphibians all breathe by gills 

 during some period of their lives, and some always- 

 reptiles never. 



Fig. 257. — Head and gills of Necturus. 



