THE FROG. 



Ill 



the air. Animals thus gifted with what really constitute 

 two modes of life are said to be amphibious. 



We must understand the meaning of this word am- 

 phibious. An accomplished diver can spend quite an 

 interval of time under water. The otter, the seal, and 

 some other animals can re- 

 main submerged several min- 

 utes without inconvenience. 

 However, neither the diver, 

 the seal, nor the otter is an 

 amphibian; while they can 

 remain a short time under 

 water, they cannot do so in- 

 definitely. While they are 

 submerged, pulmonary res- 

 piration is completely sus- 

 pended, and their stay under 

 water cannot be prolonged 

 beyond the time, always very 

 limited, in which arrested 

 respiration is followed by 

 death. 



It is not so with true am- 

 phibians. If a frog be placed 

 in a vessel full of water and 

 kept at the bottom so that 

 it cannot possibly come to 

 the surface, it will neverthe- 

 less continue to live indefinitely without appearance of 

 discomfort as long as it is furnished with food, provided 

 the water be aerated or frequently changed. It lives 

 under water as naturally and as easily as a fish ; it also 

 lives in air as comfortably as do terrestrial animals : it is 



ARTERIAL SYSTEM OF AMPHIBIAN. 

 ra, right auricle ; la, left auri- 

 cle ; v, ventricle ; ca, conus arte- 

 riosus ; c, carotid arteries ; aa, 

 aortic arches ; ao, dorsal aorta ; 

 pa, pulmonary artery ; pv, pul- 

 monary vein; ra, receives ve- 

 nous blood from body ; both the 

 pulmonary arteries enter la. 

 (After Nuhn.) 



