202 ZOOLOGY. 



11. Nervous System. The central nervous system of 

 Amphioxus occupies an exactly corresponding position to 

 that of our other Vertebrate types. It consists of a spinal 

 cord whose anterior end is but feebly dilated into a brain 

 which shows no sign of the three primary vesicles or other 

 familiar structures. The spinal cord has as usual a central 

 canal, but there is no ventral fissure, and the dorsal fissure 



apparently opens into the central 

 canal. Although both nerve-cells 

 and fibres are found in the cord 

 there is no clear segregation to 

 form white and grey matter. 

 Although there are both dorsal 

 ^-<e*r/<MA*u*EvK sensory and ventral motor roots 

 to the spinal nerves, these never 

 unite to form mixed nerves the 

 dorsal run straight out to the 

 skin between the myomeres, while 

 the ventral run into the myomeres 

 (figs. 104 and 109). Thus they 

 alternate in position as do those 

 of the dogfish. But in addition 

 to this they are affected by the 

 alternation of the myomeres; and 

 so a dorsal nerve on the right side 

 ; ^ comes off opposite a ventral one on 



the left, and vice versa. Again the 

 Fig. 109. ANTERIOR PORTION OF dorsal nerves have no ganglia, their 



SPINAL CORD OF AMPHIOXUS. fibreg bek)nging to cellg within the 

 Dorsal view. (From Willey.) gpinal CQrd . and the ventral nerves 



are divided into bunches of fibres. 



From the brain two pairs of sensory nerves come off : 

 these cannot be compared very well with any of the cranial 

 nerves of the dogfish, nor can any of the latter be clearly 

 recognized in the spinal nerves of Amphioxus. There are 

 no sympathetic ganglia.. 



12. Sense Organs. The skin, especially in the mouth- 

 region, is abundantly furnished with sensory cells epidermal 

 cells with a stiff projection after the fashion of the rabbit's 



