NERVOUS SYSTEM 



127 



The Central Nervous System is dorsal and tubular as in 

 Vertebrates, and lies in a connective-tissue sheath immediately 

 above the notochord (Figs. 71, etc., and 80, A). Posteriorly it 

 tapers to a fine point a little in front of the end of the noto- 

 chord, but anteriorly it ends abruptly some distance behind the 

 anterior extremity of the notochord. The central canal is 

 connected with the dorsal surface by a median longitudinal 

 cleft (Fig. 80, C), and at the anterior end it dilates to form the 

 cerebral vesicle (c.ii) with which two simple sense-organs, an eye- 

 spot (e) and an olfactory pit (olf), are connected. A patch of 

 ciliated epithelium in the floor of the vesicle has been described 



FIG. 79. Nephridia. A, por- 

 tion of a nephridium of 

 Phyllodoce, a marine Poly- 

 chaete, for comparison 

 with B, portion of a 

 nephridium of Amphioxus. 

 These figures show the 

 solenocytes with their 

 flagella projecting through 

 the long tubes into the 

 lumen of the excretory 

 organ, and demonstrate 

 the essential similarity of 

 the nephridia of Am- 

 phioxus with those of 

 Polychaete worms (after 

 Goodrich). 



as an " infundibular - organ." There is also a surface dilata- 

 tion of the dorsal cleft behind the cerebral vesicle (dil). The 

 nervous system as far back as this point may be regarded 

 as the brain, though scarcely distinguishable externally (Figs. 

 71 and 80, A) from the spinal chord behind. From this 

 " brain " arise two pairs of " cranial " nerves, the first (I.) from 

 the anterior end, and the second (II.) from the dorsal surface of 

 the cerebral vesicle ; both are in front of the first myotomes of 

 the body, and supply the pre-oral snout with nerves. 



The spinal cord gives off a large number of spinal nerves 

 segmentally arranged, but, like the myotomes, not opposite and 

 symmetrical on the two sides, but placed alternately (Fig. 83). 

 Moreover, the spinal nerves arise on each side at two levels, 

 there being a more dorsal series each arising by a single root and 



