3o8 



THE LANCELET. PHYLUM CHORDATA 



pair of nerves arise from the lower side of the anterior end of 



the cord, the second pair from the dorsal surface behind the 



cerebral vesicle. These pairs are symmetrical. 

 They are distributed to the epidermis of 

 the structures round the mouth and are 

 sensory in function. The remaining nerves 

 are not symmetrical, but alternate with one 

 another on the two sides, in correlation with 

 the alternation of the myomeres. Each 

 corresponds to a dorsal or a ventral root 

 of a spinal nerve of the mammal, the 

 ventral roots being placed in front of the 

 dorsal. The roots do not join, the ventral, 

 which are groups of slender rootlets, passing 

 direct to the myotomes, and the dorsal, 

 which are compact, passing in the septa 

 between the myomeres to the epidermis, 

 to the non-segmented ventral muscles and 

 to the gut. The ventral roots are motor, and 

 the dorsal roots mixed ; the latter have no 

 ganglia, their fibres being in the condition 

 of the afferent fibres in a worm such as 

 Nereis (Fig. 119, B), that is, having their 

 nerve cells at the periphery of the body. 

 They are said also to supply the alimentary 

 canal. The sense organs are few and simple. 

 Supposed tactile cells bearing short, stiff 

 processes are scattered among the ordinary 

 ectoderm cells, especially at the front end 

 of the bodv and around the mouth, and the 

 animal responds to touch by swimming 

 backwards or forwards as the front or hind 

 end is stimulated. Possible chemoreceptors 

 are the olfactory pit and cells of the oral 

 cirri. The mass of pigment which lies in the 

 front wall of the cerebral vesicle probably 

 protects that part of the cord from light ; 



more posteriorly in the lower part of the nerve cord are cells 



which appear to be sensitive to light. 



-The 

 the 



front 

 nerve 



Fig. 234.- 

 part of 

 cord of Branchio- 

 stoma, seen from 

 above. 



c.c, Central canal ; c.v., 

 cerebral vesicle ; pg., 

 anterior pigment spot ; 

 pg'., pigment spots in the 

 floor of the cord ; v.r., 

 ventral root ; d.r., dorsal 

 root ; /, //, first two 

 pairs of nerves. 



