NERVE CORD. 



17 



of the myotome from septum to septum. The other striated 

 muscles are very similar to the lateral muscle in structure, but 

 the cross-striation is less marked, and they are not segmentally 

 arranged, nor derived from the myotomes of the embryo. The 

 transverse muscles extend from the ventral end of the lateral 

 muscles to the middle line of the floor of the ventral groove, 

 where they are inserted into a median connective tissue septum 

 (Fig. 10). The unstriped muscle confers contractility on the 

 walls of the intestine and larger blood-vessels. It is exceedingly 

 inconspicuous and thin. It is doubtful indeed if it really exists 

 as a distinct tissue. 



The nervous system. The 

 nerve-cord (cerebro - spinal 

 cord) contains in its ventral 

 portion a small circular cen- 

 tral canal, which extends as a 

 narrow fissure to the dorsal 

 summit of the cord (Fig. 6). 

 This canal is lined by colum- 

 nar epithelial cells, some of 

 which are continued into the 

 substance of the cord as sup- 

 porting fibres, while others 

 may have the form of nerve 

 cells. The cells lining the 

 dorsal part of the canal are in 

 close contact, so that the 

 cavity here is virtual. The 



nerve cells are for the most part placed in the central part of 

 the cord, and some of them are of giant size,* and extend 

 right across the fissure-like part of the central canal. On the 

 ventral side of the canal, at short (metameric) intervals along 

 the whole length of the cord behind the second myotome, are 

 small groups of black pigmented cells, f These structures are 

 probably sensitive to light. There is also a pigment spot, 

 commonly called the eye, and placed at the front end of the 

 cord in the anterior wall of the cerebral vesicle. 



* For the arrangement of these giant nerve-cells and of the giant fibres 

 which issue from them, we refer the reader to Rohde, in Zool. Beitrage, 2, 

 1888, p. 169. 



f Hesse, Z. f. w. Z., 63, 1898, p. 456. 



Z. ii. C 



FIG. 6. Transverse section of the spina I 

 cord of Amphioxus (after Rohde). gz 

 nerve cells ; stf supporting fibres ; ck cen- 

 tral canal. 



