398 



CEPHALOCHORDA 



[CH. 



is embedded are scattered at intervals all along the nerve-tube. 

 It has been proved that Amphioxus is sensitive to light, but that it 

 is equally sensitive if the front end containing the cerebral vesicle 

 with its eye-spot be cut off. Therefore it is held, and in our 

 opinion rightly, that all the pigmented spots in the nerve-cord are 

 seats of light-sensation. In the wall of the nerve-tube are to be 

 found two kinds of nerve-cells, viz., (a) ordinary small nerve-cells, 

 the processes of which soon pass outwards into the peripheral 



FIG. 194. Transverse section through the spinal cord of AmpMoxus in the 

 middle region of the body. After Kohde. 



a. Giant fibre. c.c. Central canal. g.f 1 , Giant nerve-fibres, which traverse 

 the spinal cord from before backwards. #./ 2 . Giant nerve-fibres, which 

 traverse the spinal cord from behind forwards. m.p. Muscle-plates, i.e. 

 terminations of the nerve-fibres on the muscles. 

 n.f. Longitudinal nerve-fibres cut across. 

 sh. Sheath of nerve-cord. 



m.r. Motor nerve-fibres. 

 s.f. Supporting cells. 



nerves, and (b) very large nerve-cells, the processes of which extend 

 almost throughout the entire length of the nerve-tube. The 

 processes of the latter kind of cell are called "giant fibres" (g.f 1 

 and g.J", Fig. 194) : they appear to have to do with coordinating 

 the muscular movements of the animal. Besides the nerve-cells, 

 as in all nervous systems, there are a certain number of supporting 

 cells (s.f, Fig. 194). In the embryo of Amphioxiis the whole 

 wall of the nerve-tube consists of a single layer of cells, all of 



