XIII 



PHYLUM CHORDATA 



47 



narrow anterior ends probably communicate with the ccelome. 

 There are also groups of columnar excretory cells on the floor of 

 the atrium. 



Nervous System.- -The central nervous system is a rod-like 

 organ the neuron or dorsal nerve-tube (Fig. 697, A, n. ; B. neu., 

 Figs. 698, 699), contained within and completely filling a median 

 longitudinal neural canal which lies immediately above the noto- 

 chord. It is roughly triangular in transverse section : anteriorly 

 it ends abruptly some distance behind the anterior end of the 





FIG 



"01. Amphioxus lanceolatus. A, nephridium of the left side with part of the 

 wall of the pharynx. (From Willey, after Boveri.) 



notochord, while posteriorly it tapers to a point over the hinder 

 end of the latter. It is traversed bv an axial cavity, the neuro- 



/ / 



cede (Fig. 698, cent, c.), connected with the mid-dorsal region by 

 a longitudinal cleft. At the fore-end of the nerve-tube the 

 neuroccele becomes greatly dilated, forming a considerable cavity, 

 the encephaloccde or cerebral ventricle (Fig. 698, en. cos., Fig. 702, 

 cv.\ and a little behind this the dorsal fissure widens out above 

 to form a trough-like dorsal dilatation (dil.) covered only by the 

 delicate connective tissue sheath which invests the whole nerve- 



