CEPHALOCHOEDA 



139 



The rest of the mesoderm is relegated to the formation of 

 the connective tissues and the blood system, and is but slightly 

 developed. 



Circulatory System. The blood is a colourless fluid, or 

 plasma, without corpuscles, which is circulated in vessels and 

 sinuses, of which the heart and the aortae alone possess a wall. 

 The disposition of the vessels, however, is of great interest. 

 The heart is little more than an S-shaped bend of the vessel at 

 the postero-ventral end of the pharynx. The blood is conducted 

 forwards from the heart in a ventral aorta which lies below 

 the endostyle. The ventral aorta gives off afferent branchial 

 vessels which run up each gill, from which they emerge dorsally 



Dorsal aorta 



Ant, card. 



Cuvierian 



vein Dorsal aorta Posterior card. 



Caudal 

 artery 



Ventral aorta Hepatic vein Portal Parietal Subintestinal Caudal vein 



vein vein . vein 



FIG. 65. Amphioxus. Diagram of blood circulatory system. 



as efferent branchials. The efferent branchials run into the 

 dorsal aortae, paired vessels which convey the blood forwards 

 to the head and posteriorly unite behind the pharynx to form 

 a median dorsal aorta, and that in turn runs into the tail as 

 the caudal artery. 



The caudal vein returns the blood from the tail, and on 

 entering the body from the tail splits into two vessels : (1) the 

 subintestinal and (2) the right posterior cardinal veins. (1) The 

 subintestinal vein forms a sinus system around the intestine. 

 This plexus of the intestine gives off anteriorly a vein, the 

 hepatic portal, which carries the blood to the hepatic caecum, 

 around which it divides into a sinus system. The blood is 

 collected from the hepatic sinuses by a hepatic vein which 

 enters the right (the posterior) end of the heart. (2) The 

 right posterior cardinal vein runs forwards from the caudal 

 vein, and at the level of the heart it meets a similar vein from 

 the right anterior side of the body, the anterior cardinal, and 



