XIII 



PHYLUM CHORDATA 



89 



as both afferent and efferent vessels of the hepatic and renal 

 capillaries, the afferent vessels of both organs ending in capil- 

 laries after the fashion of 

 arteries. 



In the embryos of the higher, 

 or air-breathing, Craniata the 

 circulatory organs agree in 

 essentials with the above de- 

 scription, the most important 

 difference being that, as no 

 gills are present, the branches 

 of the ventral aorta do not 

 break up into capillaries, but 

 pass directly into the dorsal 

 aorta, forming the aortic arches 

 (Fig. 729, Ah.). With the ap- 

 pearance of the lungs, however, 

 a very fundamental change 

 occurs in the blood-system. 

 The last aortic arch of each 

 side give off a pulmonary 

 artery (Fig. 730, Ap.) to the 

 corresponding lung, and the 

 blood, after circulating through 

 the capillaries of that organ, 

 is returned by a pulmonary 

 vein (lv.), not into an ordinary 

 systemic vein of higher order,, 

 but into the heart directly : 

 there it enters the left side of 

 the auricle, in which a vertical . 

 partition is developed, separat- 

 ing a left auricle (A 1 ), which \ 

 receives the aerated blood from 

 the lungs, from a right auricle 

 (A), into which is poured the 

 impure blood of the sinus 

 venosus. Lastly, in Crocodiles, 

 Birds, and Mammals the ven- 

 tricle also becomes divided into 

 right and left chambers (B.), 

 and we get a four-chambered 

 heart, having right and left 

 auricles, and right and left 



ventricles : at the same time the sinus venosus ceases to exist as a 

 distinct chamber. The left auricle receives aerated blood from the 

 lungs and passes it into the left ventricle, whence it propelled 



FIG. 729. Diagram of the vascular system in the 

 embryo of an air-breathing Craniate. 



A, dorsal aorta and auricle ; Ab, aortic arches ; 

 Acd, caudal artery; All. allantoic arteries; 

 Am, vitelline arteries ; , ventral aorta ; C, C 1 , 

 carotid arteries ; D, pre-caval veinsv; Jc, E, 

 iliac arteries ; HC, cardinal veins ; KL, gill- 

 clefts ; . A. S, -S 1 , roots of dorsal aorta ; Sb, 

 sub-clavian arteries ; Sb}, sub-clavian veins ; 

 V. Ventricle ; VC, jugular vein ; Vm, vitelline 

 veins. (From Wiedersheim's Vertebrata.) 



