CIRCULATION IN TADPOLE. 249 



however, and the animal is being prepared for its new mode 

 of life, the lungs are gradually developed, and the pulmonary 

 arteries greatly increase in size ; whilst the gills, on the other 

 hand, do not continue to grow with the animal, but rather 

 shrink, from the diminished supply of blood w T hich they 

 receive. For, during this period, the communicating branches 



ap av ap 



Fig. 137. THE SAME, IN A MORE ADVANCED STATE. 



increase in size ; so that a considerable part of the blood which 

 has been transmitted into the branchial arteries passes at once 

 into the veins, and thence into the aorta, without being made 

 to traverse the gills ; its aeration being partly accomplished 

 by the lungs. This state of things is seen in fig. 137 ; where 

 ap, ap, are the enlarged pulmonary arteries ; and where the- 

 communicating branches are seen almost to form the natural 

 continuations of the branchial arteries. A condition of this 

 kind exists permanently in those Batrachia which retain their 

 gills during their whole lives, and have the lungs imperfectly 

 developed ( 87). When the metamorphosis is complete, the 

 branchial vessels altogether disappear, but the arches still 

 remain, as shown in fig. 138. The first of these arches sup- 

 plies the vessels of the head, 1 1 ; which also, however, receive 

 a branch o from the second arch. The second arch, after 

 giving off that branch, unites with its fellow to form the 

 aortic trunk av. The third arch has completely shrivelled 

 up. And the fourth arch or pulmonary artery has now 

 attained its full size, and is become the sole channel through 

 which the aeration of the blood is effected. 



