BACKBONED ANIMALS WHICH BREATHE IN AIR 423 



in ordinary fishes), but also internal nostrils, placed just within the 

 upper lip. It is clearly advantageous for a land-animal to take in 

 air through the nose, for food is often detected by smell, and this 

 device increases the chance of finding it. In a Frog the internal 

 nostrils open rather further back than in a Mud- Fish, and the 

 external nostrils are valvular. The chief agency by which air 

 is taken in and passed out is found in the muscular floor of the 

 mouth, which is moved alternately up and down. The procedure 

 adopted is somewhat as follows. The mouth being closed, its 

 floor is lowered, when air passes through the nose into the 

 mouth-cavity, after which the valvular nostrils are shut. The 

 floor of the mouth is now raised, and the air is forced into the 

 lungs. The blood in these organs having been purified to some 

 extent, the floor of the mouth is again lowered, and the impure air 

 is drawn out of the lungs into the mouth-cavity. The tip of the 

 snout is next bent down a little so as to open the external nostrils, 

 the mouth-floor is raised, and the air forced out through the nose 

 to the exterior. 



The Frog is only partially adapted to a life on land, being 

 dependent upon a damp surrounding atmosphere, and, taking- 

 advantage of this, it has retained, or perhaps reacquired, the 

 old method of breathing by means of the external surface of the 

 body as an accessory to the more specialized respiration effected 

 by lungs. 



In the snake -shaped Caecilians only one lung is properly 

 developed, for two efficient organs of the kind could not be 

 packed into the narrow body. 



It is interesting to notice, in passing, that some Salamanders 

 have specialized on quite different and very extraordinary lines 

 in regard to respiration. Their peculiarity consists in the fact 

 that they have abandoned lung-breathing altogether, for their 

 lungs are either entirely absent or else reduced to useless remnants, 

 while at the same time the structure of the heart has undergone 

 a corresponding change. How breathing is carried on under such 

 circumstances is not definitely known, but the skin, the lining of 

 the mouth and pharynx, and the lining of the intestine have all 

 been suggested as the parts which supply the place of the absent 

 lungs. The pretty little spectacled Salamander (^Salainandrina 

 perspicillatd) of Northern Italy will serve as an example of these 

 lungless Amphibians. 



