THE VASCULAR SYSTEM 35^ 



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 lishes 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 aUmentary 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 

 body-wall. These two vessels parallel one another and both connect 

 anteriorly with the heart. Venous blood from the body-wall is returned 

 to the heart by the cardinal veins, anterior and posterior. 



Amphioxus. In most details the vascular system of amphioxus 

 resembles the hypothetical ancestral system just described. But amphi- 

 oxus is heartless, and the circulation of its blood is dependent upon the 

 contractility of the walls of its main blood trunks. Blood is carried for- 

 ward beneath the pharynx by a contractile ventral blood vessel, the 

 truncus arteriosus, and distributed to the numerous gills by a series of 

 paired afferent vessels, the aortic arches. Contractile enlargements or 

 bulbils of these vessels aid in the propulsion of blood to the gills. Neph- 

 ridia similar to those of annelids are associated with the gills and pre- 

 sumably assist in the^ elimination of nitrogenous wastes. From the 

 efferent vessels the blood passes to the paired dorsal aortae, the anterior 

 extensions of which correspond to the internal carotid arteries of verte- 

 brates. Posterior to the gills the paired aortae unite to form the median 

 dorsal aorta of the trunk region. From the dorsal aorta paired inter- 

 segmental vessels are given off to the body-wall and a series of median 

 unpaired vessels to the alimentary canal. In the tail region the caudal 

 artery has intersegmental connexions with the caudal vein. Beginning 

 at the anal region blood from the caudal vein may be returned to the heart 

 either through the right postcardinal vein or by the subintestinal vein. 

 Like the hepatic portal vein of vertebrates, the subintestinal vein of amphi- 

 oxus breaks up in capillaries within the Uver. Anteriorly the hepatic 

 capillaries unite to form the hepatic vein which carries blood to the sinus 

 venosus and the truncus arteriosus. Amphioxus has no renal portal 

 system such as occurs in fishes and amphibians. Venous blood is brought 

 from the anterior part of the body by the precardinal veins and from the 



