300 PHILOSOPHY OF ZOOLOGY. 



force of these arguments, we shall continue the comparison, 

 especially as applied to the organs of respiration. 



" In the larvae of the frog and salamander, the trachea 

 opens directly into the lungs. These organs have the form 

 of two sacs, and, from being longer than the trunk, cannot 

 be extended in a straight line through it, but at the lower 

 end are folded a little from one side of the abdomen to the 

 other. So, in the siren, we see the trachea to open directly 

 into the lungs, which, as in the above-mentioned larvae, 

 says M. Cuvier, ' sont deux longs sacs cylindriques, que 

 s'etendent jusqu' a Textremite posterieure de Tabdomen, et 

 se replient meme alors en avant.' But, in the proteus, nei- 

 ther do the supposed lungs reach to the pelvis, nor does the 

 supposed glottis open into the air-bladders, but issue in a 

 cavity which communicates with the air-bladders by two 

 long conduits. Thus, then, the structure of the branchial 

 arches, the distribution of the bloodvessels, and the form 

 and size of the lungs in the proteus, differ entirely from the 

 corresponding organs in the siren and larvae of the salaman- 

 der. 



" If, farther, we consider the mode in which frogs and sa- 

 lamanders respire air, and compare it with that of the pro- 

 teus, we shall obtain still further evidence of the differences 

 subsisting between them. All zoologists, including M. Cu- 

 viEE, now admit that frogs first receive air into the mouth 

 through the nostrils only, and from thence force it into the 

 lungs by an action resembling deglutition. But neither the 

 proteus nor the siren are able to respire in this manner ; 

 for the nostrils in the former do not open into the mouth, 

 but beneath the upper lip ; and in the siren, ' les narines, 

 simplement creiisees sur les cotes du museau, ne penetrent 

 point dans la bouche,'' says M. Cuvjer. Neither do these 

 animals respire au- in the manner of serpents, for they are 

 both destitute of ribs. When also the proteus takes air in- 



