236 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the tissues of different animals differ in the amount 

 they require. 



We may be especially convinced of the truth of this 

 dictum of Bert's by the study of the respiratory arrange- 

 ments of the Amphibia. If an adult frog is placed in a 

 dry atmosphere it speedily dies ; in other words, respira- 

 tion in a frog is cutaneous as well as pulmonary, and a 

 frog may be deprived of its lungs and continue to live ; 

 so, again, the axolotl may have both gills and lungs 

 removed and yet continue to live. But if these 

 experiments are made in summer death soon super- 

 venes ; in other words, the skin becomes more dry 

 owing to the larger amount of moisture which can be 

 taken up by an equal volume of warmer air, and is 

 unable to take up enough oxygen to suffice for the 

 needs of the organism. 



In all Amphibia the gills are at first external, or 

 project, under various forms, from the sides of the 

 body ; there are ordinarily three pairs present, which 

 are placed one above the other ; among the Urodela, 

 Menobranchus (Fig. 100) and Proteus appear to 

 retain the gills throughout life; in Menopoma and 

 Amphiuma the gills disappear, but one or two gill 

 clefts persist ; in the rest of the Urodela the gills dis- 

 appear completely. In the Amira the external are 

 soon replaced by internal gills, which, on the cessation 

 of the tadpole stage, disappear, and the clefts, which 

 had been covered by an opercular membrane, close up 

 entirely. The bell-shaped gills of ISTotodelphys lead to 

 the branchial vesicles which have been found in the 

 Csecilise (Peters). 



In addition to or in place of the gills, all Amphibia 

 have a paired outgrowth from the esophagus, which 

 lies on the ventral surface, and is provided with 

 special blood-vessels coming directly from and return- 

 ing at once to the heart. 



These outgrowths are known as the lungs, and 



