THE LANCELET 



337 



chord, giving branches to the gut and 

 body-wall. From these blood is 

 collected by a subintestinal vein, which 

 carries it to the liver and there breaks 

 up into a hepatic plexus. A hepatic 

 vein conveys the contents of this 

 plexus to the ventral aorta. Com- 

 parison of this circulation with that 

 of the dogfish, presently to be de- 

 scribed, will show that the general 

 course of the blood is similar in 

 the two cases. It will be seen that 

 the direction of flow in the ventral 

 and dorsal vessels of Amphioxus 

 is opposite to that in the worm 

 and the same as that of the fish, 

 that its gills are supplied in the 

 same way as in the fish, and that 

 there is in both a hepatic portal 

 system. 



The body contains a number of 

 lymph spaces. Some of these (as 

 those in the fins and certain spaces 

 among the muscles) are said to be 

 ccelomic. Others, such as the meta- 

 pleural canals of the adult, may pos- 

 sibly be hsemoccelic. 



The position of the nerve, cord 



has been described. It 



Nervous i s roughly triangular in 



System and D J . ■ u ■ 



sense organs, transverse section, being 

 flattened on its under 

 side, ends abruptly in front at the 

 level of the first myotome, 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 lined by an 



■c.c. 



t^ 



i 3fc 



Fig. 244. — The front 

 part of the nerve 

 cord of Amphioxus, 

 seen from above. 



CO., Central canal; c.v., 

 cerebral vesicle; $.g., 

 anterior pigment spot ; 

 Psf »\ pigment spots in 

 the floor of the cord ; 

 v.r. } ventral root — the 

 corresponding dorsal root 

 lies immediately hehind 

 it ; I., first pair of nerves. 



