THE VASCULAR SYSTEM 265 



system of vertebrates has been more recently suggested. Such a sug- 

 gestion, however, is obviously based upon erroneous assumptions. The 

 facts accord better with the assumption that both blood-vascular and 

 lymphatic systems have had a common origin from the primitive lacunar 

 systems of invertebrates. While in the invertebrates the circulatory 

 system has remained partly open, in the vertebrates on the other hand the 

 circulatory system, both blood-vascular and lymphatic, has become 

 wholly closed. 



The vascular system of annelids is fundamentally like that of chordates. 

 In both groups occur two main longitudinal vascular trunks, one above 

 and one below the aHmentary canal, connected with one another by aortic 

 arches around the pharynx. In the earthworm there are five pairs of 

 aortic arches. It is true that the direction of flow of blood in annelids is 

 the reverse of that in vertebrates. But this difference in blood flow is 

 nullified if the dorsal and ventral sides of the worm are reversed. In spite 

 of the necessity of inverting the worm, proponents of the annelid hypothesis 

 of vertebrate ancestry have stressed the similarity of the vascular systems 

 of the groups as strong support of their views. The absence of a heart 

 in annelids and its presence in vertebrates is not a serious objection 

 to this view, since the dorsal blood-vessel of annelids is contractile through- 

 out its length and it may be reasonably assumed that this contractile 

 function is concentrated and localized in the vertebrate heart. 



Such a diagram of the hypothetical primitive vertebrate blood- vascular 

 system as is shown in Fig. 243 is based, however, not on the assumption 

 of an anneUd ancestry of chordates but upon evidence from comparative 

 anatomy and embryology. While the circulation of blood in the ancestral 

 chordate was probably due, as in annelids, to the contractility of the walls 

 of the blood-vessels, a contractile heart such as is found in all vertebrates 

 is added to the diagram. Blood is pumped by the heart towards the head 

 through a median ventral truncus arteriosus from which pass the series of 

 aortic arches, which connect around the pharynx with the median dorsal 

 aorta. In fishes and in Amphioxus, the aortic arches are divided, by a 

 network of capillaries in the gills, into ventral afferent and dorsal efferent 

 arteries. The carotid arteries carry blood forward to the head while the 

 dorsal aorta carries it posteriorly to the trunk and tail, giving off metameric 

 intersegmental arteries to the body-wall, and median unpaired splanchnic 

 vessels to the alimentary canal. In the tail, venous blood is carried 

 towards the heart by the caudal vein. In the primitive circulation, the 

 caudal vein is assumed to be connected by intersegmental vessels with the 

 caudal artery. When the caudal vein reaches the region of the anus it 

 encircles the alimentary canal. From this point blood may return to the 

 heart either by a subintestinal vein (which also collects blood from the 

 intestine) or by an abdominal vein which extends along the median ventral 



