92 



ZOOLOGY 



SECT. 



To sum up : The circulatory organs of the branchiate Craniata 

 consist of (a) a muscular organ of propulsion, the heart, provided 

 with valves and driving the blood into (6) a set of thick-walled, 

 elastic, afferent vessels, the arteries, from which it passes into (c) a 



network of microscopic vessels 

 or capillaries which permeate 

 the tissues, supplying them with 

 oxygen and nutrient matters and 

 receiving from them carbonic 

 acid and other waste products : 

 from the capillary network the 

 blood is carried off by (d) the 

 veins, thin-walled, non-elastic 

 tubes by which it is returned 

 to the heart. Thus the general 

 scheme of the circulation is 

 simple: the arteries spring from 

 the heart, or from arteries of a 

 higher order, and end in capil- 

 laries ; the veins begin in capil- 

 laries and end in vessels of a 

 higher order or in the heart. 

 Actually, however, the system 

 is complicated (a) by the inter- 

 position 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 respira- 

 tory capillaries, the efferent 

 arteries taking their origin in 

 those capillaries after the manner 

 of veins ; and (6) by the inter- 

 position of two important blood- 

 purifying organs, the liver and 

 the kidney, in the course of the 

 returning current, as a result of 



FIG. 783. Diagram .of the vascular system in which W6 have Veins acting as 

 the embryo-of an air-breathing Craniate. 



A. dorsal aorta and auricle ; Ab. aortic arches ; both afferent and efferent vessels 

 Acd. caudal artery ; All. allantoic arteries ; e ,-L T, i i 



the heatic and renal 



both 



ending 



in 



e 



Am. vitelline arteries ; B. ventral aorta ; c, ci. Of 

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

 iliac arteries ; HO. cardinal veins ; KL. gill- 



clefts ; RA, S, S l . roots of dorsal aorta ; Sb. of o Organs 



subclavian arteries ; Sb 1 . subclavian veins ; .,, . j,. . , < i 



V. ventricle ; VC. jugular vein ; Vm. vitelline Capillaries after the f ashion of 

 veins. (From Wiedersheim's Vertebrata.) arteries 



In the embryos of the higher, or air-breathing, Craniata, the 

 circulatory organs agree in essentials with the above description, the 

 most important difference being that, as no gills are present, the 



