338 SYSTEMATIC ZOOLOGY. 



and behind the gills , thus forming a large chamber outside 

 the gills which is connected with the exterior by a small 

 opening on the left side,* through which the water used 

 in breathing passes. 



In the larva the heart is two-chambered, and the blood, 

 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 

 development 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 condition in the adult is 

 brought about not so much by new formations as by 

 modifications of pre-existing structures. Compare in this 

 connection the diagrams on page 312. 



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 



* Right and left openings occur in two tropical toads (Aglossa). 

 A few forms have a median opening. 



