120 LECTURES ON BIOLOGY 



similarly adapt itself to the altered conditions and undergo 

 suitable modifications. 



Fishes breathe through gills ; hence we find in them a specific 

 circulatory system, the branchial arches, which carry the blood 

 to and from the gills. In the majority of fishes four pairs of 

 such arches are developed. In the amphibians, at any rate 

 in those that live in the water and are therefore dependent on 

 breathing through gills, the gill-arteries and gill-veins are formed 

 in a similar manner. But many amphibians for instance, frogs 

 leave the water at an adult stage and become air-breathing land- 

 animals. The gills, now of no further use, become degenerate 

 and are replaced by the lungs. 



When the gills disappear the blood-vessels which fed them 

 similarly degenerate, and thus we observe a far-reaching modifica- 

 tion. One pair of the arches is completely lost, another moves to 

 the lungs, forming the pulmonary circulation, &c. The details do 

 not concern us here, but it is important to note that even in the 

 higher and highest vertebrates the reptiles, birds, and mammals, 

 which at no stage of their development live in the water the 

 branchial circulation is present in the embryonic stage exactly as 

 in fishes and amphibians. Moreover, if we examine a human 

 embryo of an early stage we do not only find rudimentary 

 branchial arches, but also externally visible typical gill-clefts 

 (fig. 32, 3 ). We are even able to distinguish in the human heart 

 successively the fish-heart with single ventricle, auricle, and 

 venous sinus ; and the stage of the amphibian heart in which the 

 single ventricle has been divided into two sections. Later a 

 median wall is formed representing the stage which is per- 

 manently present in the reptiles. Finally, by a complete separa- 

 tion of both ventricles and other modifications, the type of the 

 mammalian heart is reached. 



We have seen that amphibians at an early stage live in the 

 water and have gills, but at an adult stage take to a terrestrial 

 life and develop lungs. In the mountainous districts of Europe 

 the little black salamander (Salamandra atrd) is very common. It 

 is distinguished from related species by the fact that its young 

 are born as air-breathing animals. Nevertheless, its embryos 

 possess w 7 ell-developed gills which, however, do not normally 

 fulfil any functions. But if we kill a pregnant female, remove 



