XIII 



PHYLUM CHORDATA 



53 



wide, backwardly-directed ends open into the atrium ; their 

 narrow anterior ends probably communicate with the coelome. 

 "re 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. 742, A, n. ; B. neu., 

 Figs. 743, 744), contained" within and completely rilling a median 

 longitudinal neural canal which lies immediately above the nofco- 

 chord. It is roughly triangular in transverse section : anteriorly 



FIG. 740. Amphioxus lanceolatus. A, ncphridium of the left side with p\rt of the 

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



it ends abruptly some distance behind the anterior end of the 

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

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

 ccele (Fig. 743, cent, c.), connected "with the mid-dorsal region by 

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

 neurocoele becomes greatly dilated, forming a considerable cavity, 

 the encephaloccele or cerebral ventricle (Fig. 743, en. cce., Fig. 747, 

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

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



E 2 



