178 AMPHIOXUS 



4. The portal veins are vessels running, in the coelom, along 



the ventral surface of the intestine : they collect the 

 blood from the intestine and carry it forwards to the 

 liver, where they break up into capillaries. 



5. The hepatic veins are vessels which collect the blood 



from the liver and carry it backwards along the 

 dorsal surface of the liver. On reaching the junc- 

 tion of the liver and intestine, they unite to form a 

 single vessel, which turns forwards along the ventral 

 surface of the pharynx and becomes the cardiac 

 aorta. 



6. A longitudinal vessel runs along the inner side of each 



atrial fold, and supplies the myotomes and reproduc- 

 tive organs. 



G. The Excretory System. 



It is not yet certain whether Amphioxus has any definite 

 excretory organs. By different writers various structures have 

 been described as excretory, but the only ones that can possibly 

 be compared with the kidneys of other vertebrates, and these 

 only very doubtfully, are the pigmented canals discovered by 

 Lankester. These are a pair of short tubes, with deeply 

 pigmented walls, placed in the twenty- seventh segment of the 

 body, opposite the hinder end of the pharynx. They He, one 

 at each side of the body, in the dorsal coelomic canals above 

 the suspensory folds of the pharynx. Each tube is attached 

 along its outer side to the body-wall, and opens behind into the 

 atrial cavity : in front it is considerably contracted, but appears 

 to open into the coelomic canal. 



H. The Nervous System. 



' The nervous system of Amphioxus consists of : (1) the 

 central portion, which, as in other vertebrates, is a tube of 

 nervous matter lying in a sheath of connective tissue imme- 

 diately above the notochord, and extending almost the entire 

 length of the body ; (2) the peripheral portion, or nerves proper, 

 which connect the central portion with the various parts of 

 the body. 



