RESIMRATION IN REPTILES. 291 



receiving air, to be employed when the animal 

 exchanges its aqnatic for its terrestrial life. It is 

 exceedingly interesting to observe that this animal 

 at first breathes by gills, which are furnished with 

 cilia,* project in an arborescent form from the sides 

 of the neck, and float in the water ; but these struc- 

 tures are merely temporary, being provided only to 

 meet the immediate exigency of the occasion, and 

 being raised at a period when none of the internal 

 organs are as yet perfected. As soon as another 

 set of gills, situated internally, can be constructed, 

 and are ready to admit the circulating blood, the 

 external gills are superseded in their office ; they 

 now shrivel, and are removed, and the tadpole per- 

 forms its respiration by means of branchiae, formed 

 on themodelof those of fishes, and acting by a similar 

 mechanism. By the time that the system has 

 undergone the changes necessary for its conversion 

 into the frog, a new and very different apparatus 

 has been evolved for the respiration of air. These 

 are the lungs, which now coming into play, direct 

 the current of blood from the branchiae, and take 

 upon themselves the whole office of respiration. 

 The branchiae, in their turn, become useless, are 

 soon obliterated, and leave no other trace of their 

 former existence than the original division of the 

 arterial trunks, which had supplied them with 

 blood directly from the heart, but which, now 

 uniting in the back, form the descending aorta. | 



There is a small family, called the Perenni- 

 hranchia, belonging to this class, which, instead of 



* It has been found that in the young- animal nearly the whole 

 surface of the body gives rise to ciliary motions : but these motions 

 disappear as the animal advances in growth. 



t See Fig. 357, p. 248. 



