AMPHIOXUS 13 



Vascular System. — Running forward under the floor of the 

 pharynx beneath the endostyle is the ventral aorta. There is 

 no specialised heart, but this aorta is contractile, and propels 

 the blood into the afferent branchial arteries which run to the 

 primary gill-bars. At the base of the bars these arteries swell 

 into little contractile bulbils and divide into the three vessels 

 which run up the bars. The secondary bars obtain blood in 

 their two vessels indirectly from the primary bars through the 

 vessels in the synapticula. Branches are sent to the excretory 

 organs (nephridia, see p. 15) which are thereby enabled to 

 extract the excretory products from the blood. 



From the gill-bars the blood is collected into the efferent 

 branchial vessels which run to the lateral dorsal aortae, one on 

 each side of the mesentery, just above the hyperpharyngeal 

 groove. Behind the pharynx they join to form the single 

 dorsal aorta, which carries blood back to the posterior regions 

 of the body. In the septa separating each pair of adjacent 

 myotomes, segmental vessels leave the aorta and distribute 

 blood locally. The blood is collected up again into the sub- 

 intestinal vessel which runs forwards beneath the intestine 

 from the hind end of the body. It breaks up into capillaries 

 in the region of the liver- diverticulum, and so forms a hepatic 

 portal system. From the liver the vessel runs forwards 

 beneath the endostyle of the pharynx as the ventral aorta. 

 There are also paired cardinal veins running in the body-wall 

 at the level of the gonads, and extending forwards in the 

 region of the pharynx and backwards to the tail. These veins 

 connect with the subintestinal vessel by transverse veins, the 

 ductus Cuvieri, on each side, which bridge across the ccelom. 

 The blood is colourless. 



Ccelom. — The relations of the ccelom are of great import- 

 ance. The myotomes of the body are separated by septa 

 (between the segments), but they do not fit the septa closely. 

 Small spaces are left which are remnants of the myoccels. 



Behind the pharynx the relations of the ccelom are quite 

 simple and typical. The gut is suspended by a dorsal 

 mesentery in a spacious splanchnoccel. In the pharyngeal 

 region, however, the relations are slightly complicated by the 



