IV NERVOUS SYSTEM 12 
The Central Nervous System is dorsal and tubular as in 
Vertebrates, and les 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 (¢.v) with which two simple sense-organs, an eye- 
spot (e) and an olfactory pit (o/f), are connected. A patch of 
ciliated epithelium in the floor of the vesicle has been described 
Frc. 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 (di/). 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 (1.) 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, hke the myotomes, not opposite and 
symmetrical on the two sides, but placed alternately (Fig. 81). 
Moreover, the spinal nerves arise on each side at two levels, 
there being a more dorsal series each arising by a single root and 
