$2 



structure of their lungs, and the capacity of these to 

 receive and contain air. In the frog and toad, the 

 lungs consist of two large membranous bags, divided 

 into a great number of vesicles, over which the blood- 

 vessels are minutely distributed ; and, in the snake, 

 viperj and many others, the lungs are continued 

 down through the whole belly, in form of two bags. 

 Many animals also jflr ttrin ifttt\ as the otter and por- 

 poise, whose lungs are constituted like those of man, 

 can live a considerable time under water without 

 breathing : and this power is much improved by 

 habit. This is the case even in the human subject ; 

 for, while in ordinary persons, suffocation begins to 

 take place in about half a minute when the body is 

 submersed in water, those who dive for pearls, co- 

 rals, cc. are said, by long practice, to be able to 

 prolong this period to several minutes, being able to 

 keep under water as long as the seal, porpoise, and 

 the amphibia. 



67. Of all the foregoing animals, therefore, which 

 by naturalists have been placed in the class amphibia, 

 none can be said to be truly amphibious, or to possess 

 the faculty of supporting life, for an indefinite length 

 of time, either in water or in air. This faculty belongs 

 only to the syren, an animal said to be furnished 

 both with lungs and gills. Something of the same 

 sort may, indeed, be attributed to the frog at diffe- 

 rent periods of its existence ; for we are told, that, 

 during the first fourteen days of its life as a tadpole, 

 it has only gills projecting like fins : that, by the 

 thirty-sixth day, these are taken into the jaws, and 

 form four rows of gills on each side, like those of 

 fishes : and that, during this time, the lungs, as in the 



