VASCULAR SYSTEM 



307 



into a hepatic plexus. A hepatic vein, which is joined by a pair of 

 vessels (ductus Cuvieri) from the body-wall, conveys the contents 

 of this plexus to the ventral aorta. 



The body contains a number of lymph spaces. Some of these 

 (as those in the fins and certain spaces among the muscles) are 

 of coelomic origin. Others, such as the metapleural canals of the 

 adult, may possibly be haemocoelic. There is no evidence that 

 the gill slits have any respiratory function, and at least when the 

 animal is swimming, most of the oxygen is probably taken up 

 through the skin. 



NERVOUS SYSTEM AND SENSE ORGANS 



The position of the nerve cord has been described. It is roughly 

 triangular in transverse section, being flattened on its under 



Z ateral dorsal aortas. 

 Hepatic vein. 



Five stumps ^Jephndial 



oftranchi ais ■ plexus. Caroti a arteries 



Moniiiform 

 artery at 

 vMase of velum. 



Branchial 



\ Vessels 

 ^ of cirri 



Ventral Pharynx. ^''^'^^^' 

 aorta. 



Fig. 233. — A diagram of the vascular system of Branchio stoma, from the right 



side, and slightly from above. 



side, ends abruptly in front at the level of the first myomere, 

 and behind tapers to a point over the hind end of the notochord. 

 There is no ventral fissure, but a deep dorsal fissure, which is 

 clearly due to the closure of a tube, part of which remains as 

 the minute central canal. This tube is Hned by an epithelium, 

 around it lie nerve cells, and the remainder of the cord is composed 

 of non-medullated fibres. At the anterior end the canal widens 

 out into a cerebral vesicle (Fig. 234), which in the larva communi- 

 cates by a pore with a ciliated funnel known as the olfactory pit, 

 on the dorsal surface of the left side of the body. In the adult this 

 opening is lost, though the pit remains, and is perhaps a chemo- 

 receptor. A ciHated depression of the floor of the vesicle perhaps 

 corresponds to the infundibulum of a vertebrate animae. The first 



