CIRCULATION IN BATRACHIA. 
247 
systemic veins, is transmitted by its ventricle into a trunk, 
which subdivides into four or five pairs of branches or arches 
(fig. 135). These branches run along the fringes which form 
the gills of the fish, and send a minute vessel into every one 
of their filaments (§ 312). Whilst passing through this 
vessel, the blood is submitted to the influence of the air 
diffused through the water, to which the gills are freely 
exposed, and is thus aerated; and it is then collected from 
the several filaments and fringes, into a single large trunk, 
analogous to the aorta of the higher animals, by which the 
whole body is supplied with arterialized blood. After circu¬ 
lating through the system, the blood returns to the heart in a 
venous condition, and again goes through the same course. 
This course is represented in a simple form in the diagram, 
fig. 131; and it will be seen, on a little consideration, that 
it does not differ from that which exists in Animals with a 
complete double circulation, in any other essential particular 
than this,—that there is no systemic heart to receive the blood 
from the gills or aerating organs, and to convey it to the body 
at large. But, though all the blood must necessarily pass 
through the gills before it can again proceed to the body, it 
does not follow that the blood should be as completely aerated 
as in Beptiles, in whose circulation there is a mixture of 
venous and arterial blood; for the exposure of the blood to 
the small quantity of air which is diffused through water is 
not nearly so effectual as its direct exposure to air. 
287. There is a group of animals which forms the transition 
between Fishes and Beptiles; some of them being Fishes at 
one part of their lives, and Beptiles at another ; whilst others 
remain, during their whole lives, in a condition intermediate 
between the two groups. Of this group (§ 86), the common 
Frog is a familiar example. In the Tadpole state, it is essen¬ 
tially a Fish, breathing by means of gills, and having its cir¬ 
culation upon a corresponding plan; but after it has gone 
through its metamorphoses, it breathes by lungs, its heart 
acquires an additional auricle, and the whole plan of the circu¬ 
lation is changed, so as to become comformable to that of the 
true Beptile. This process takes place, not suddenly, but by 
progressive stages; and as these are extremely interesting, 
they will now be briefly described. In fig. 136 we have a 
representation of the circulating apparatus of the Tadpole in 
