BATRACH1A. 51 



passing forward from it, traverses afferent and efferent 

 branchial arteries, as in fishes, and is collected, as in those 

 forms, in a dorsal aorta. With the loss of gills and the de- 

 velopment of lungs the gill circulation changes. The first 

 arterial arch becomes converted into the carotid artery, 

 supplying the head ; the second, the aortic arch, connects the 

 heart with the dorsal aorta; the third dwindles and usually 

 disappears; while the fourth, the pulmonary artery, carries 

 blood to the lungs and skin. As will be seen, the embryonic 

 circulation is like that of the fishes, but the different con- 

 dition in the adult is brought about not so much by new 

 formations as by modifications of pre-existing structures. 



In the larva the heart pumps only venous blood, as in 

 the fish. With the development of lungs and the division 

 of the single auricle into two, different conditions occur. 

 Blood from the body (venous) is poured into the right 

 auricle, and blood from the lungs (arterial, because in the 

 lungs it comes into contact with the air) into the left. 

 From the auricles the blood goes to the single ventricle, 

 and thence through the arterial trunk to head, body, and 

 lungs. So at first sight it would appear as if all parts 

 must receive a mixture of arterial and venous blood, but 

 this is not exactly the case. By means which cannot 

 be described here the purest arterial blood goes to the head, 

 the next to the aorta, while the venous blood is sent to the 

 lungs. 



In the larvae of the frogs and toads the mouth is small 

 and the horny jaws are adapted to scraping small plants 

 from submerged objects. Correlated with this vegetable 

 food is an extreme length of intestine, it being a noticeable 

 fact that herbivorous animals require a longer digestive 

 tract than carnivorous forms. 



In the larvae there is also a well-developed lateral-line 



