xvm] 



NERVOUS SYSTEM 



399 



which abut on the cavity of the tube ; many of these cells become 

 afterwards transformed into small round nerve-cells, and recede from 

 the cavity, assuming a more peripheral position : but others retain 

 their connection with the cavity and become drawn out into fibre- 

 like supporting cells. From the nerve-cord are given off two kinds 

 of nerves, but not at the same level, so that in a transverse section 

 one kind only is seen. These are: (1) sensory nerves, going 

 directly to the skin and having a dorsal origin ; (2) motor nerves, 

 going to the myotomes. The nervous tube and the alimentary 

 canal at first both reach to the extreme posterior end of the body 

 and here are connected by 

 a vertical tube, the neur- 

 enteric canal. On the 

 course of this tube the 

 anus is formed. As de- 

 velopment proceeds the 

 anus slowly shifts forwards 

 and the neurenteric canal 

 becomes a solid string of 

 cells and disappears. Thus 

 is initiated the formation 

 of a tail, by which term is 

 denoted a portion of the 

 body devoted entirely to 

 locomotion and freed from 

 all part of the gut, being 

 filled only with muscles. 

 The tail of Amphioxus 

 acquires only a very limited 

 development, but it soon 

 becomes surrounded by a 

 tail fin, at first merely 

 made up of the enlarged 

 skin cells, but soon becom- 

 ing a flap containing gela- 

 tinous material. A similar 

 fold along the middle line 

 of the back forms the dor- 

 sal fin, in which, in the larva, there are a series of metamerically 

 arranged cavities lined by distinct epithelium, and although more 

 numerous than the myotomes they are probably derivatives of 



FIG. 195. Amphioxus. Nephridium of the 

 left side, with the neighbouring portion of 

 the pharyngeal wall, as seen in the living 

 condition. The round bodies in the wall 

 of the tubule represent carmine granules. 

 Highly magnified. After Boveri. 



