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 which they 
receive. For, during this period, the communicating branches 
1 t o t 
3 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, 11; 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 trank 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. 
