BLOOD-VESSELS. 153 



The bulbus or conus arteriosus is prolonged into the bran- 

 chial artery which soon divides, sending off a branch to each 

 branchial arch. On returning from the respiratory organ the 

 branchial veins assume the structure and functions of arteries. 

 Several branches are sent off to different portions of the head 

 and to the heart, but the main trunks unite to form the great 

 artery which carries the blood to the viscera and all the parts 

 of the trunt and tail, and which, therefore, represents the 

 aorta of higher animals. 



In the majority of Teleosteans the aorta has proper walls 

 formed by its own membranes, but in the Sturgeons it is 

 independent at its commencement only, and replaced by a 

 canal formed by hsenial elements of the vertebral column, 

 and clothed inside with a perichondrium. In many Chon- 

 dropterygians and some Teleosteans (Esox, Clupea, Silurus), 

 the aorta possesses its own firm membranes along its ventral 

 side, dorsally being protected by a very thin membrane only, 

 attached to the concavity of the centra of the vertebrae. 



The circulatory system of Branchiostoma and of the Dipnoi 

 shows essential differences from that of other fishes. 



Branchiostoma is the only fish which does not possess a 

 muscular heart, several cardinal portions of its vascular 

 system being contractile. A great vein extends forwards along 

 the caudal region below the notochord, and exhibits con- 

 tractility in a forward direction ; it is bent anteriorly, passing 

 into another tube-like pulsatile trunk, the branchial heart, 

 which runs along the middle of the base of the pharynx, 

 sending off branches on each side to the branchiae ; each of 

 these branches has a small contractile dilatation (hulbillus) at 

 its base. The two anterior branches pass directly into the 

 aorta, the others are branchial arteries, the blood of which 

 returns by branchial veins emptying into the aorta. The 

 blood of the intestinal veins is collected in a contractile tube, 

 the portal vein, situated below the intestine, and distributed 



