XIII 



PHYLUM CHORDATA 



spring from the heart, or from arteries of a higher order, and end 

 in capillaries ; the veins begin in capillaries and end in vessels of a 

 higher order or in the heart. 

 Actually, however, the system 

 is complicated (a) by the in- 

 terposition of the gills in the 

 course of the outgoing current, 

 as a result of which we have 

 arteries serving as both afferent 

 and efferent vessels of the re- 

 spiratory capillaries, the effer- 

 ent arteries taking their origin 

 in those capillaries after the 

 manner of veins ; and (b) by 

 the interposition of two im- 

 portant blood-purifying organs, 

 the liver and the kidney, in 

 the course of the returning 

 current, as a result of which 

 we have veins acting as both 

 afferent and efferent vessels of 

 the hepatic and renal capil- 

 laries, 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. 774, Ab.). With the ap- 

 pearance of the lungs, however, 

 a very fundamental change 

 occurs in the blood-system. 

 The last aortic arch of each 

 side gives off a pulmonary 

 artery (Fig. 775, 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 



Fi<;. 774. Diagram of the vascular system in the 

 embryo of an air-breathing Craniate. 



A, dorsal aorta and auricle ; Al>, aortic arches ; 

 Acd, caudal artery ; All. allantoic arteries ; 

 Am, vitelline arteries ; B, ventral aorta ; c, c 1 . 

 carotid arteries ; D, precaval veins ; Ic, E, 

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

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

 subclavian arteries; &i, subclavian veins; 

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

 veins. (From Wiedersheim's Vertebmta.) 



