CIKCULATION IN BATEACHIA. 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 Reptiles, 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 Eeptiles ; some of them being Fishes at 

 one part of their lives, and Reptiles 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 Reptile. 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 



