Ml[ 



PHYLUM CHORD ATA 



53 



wide, backwardly-diivctod ends ojxmi into the atfiuiii ; tJicii- 

 narrow anterior ends probably coiuniunicate with the coelome. 

 There arc also groups of cohmniar excretory cells on the floor of 

 tlie ati-inni. 



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

 organ, the lunrvn or dorsal ncrvc-tuhc (Fig. 742, A, n. ; 11 neAi,., 

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

 longitudinal yicural canal which lies iinniediately above the noto- 

 chord. It is roughl}' triangular in transverse section : anteriorly 



Fio. 740. — AmphioXUS lanceolatUS. A, nephridium of the left side witli part of the 

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



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, cait. c), connected with the mid-dorsal region by 

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

 neurocoele becomes greatly dilated, forming a considerable cavity, 

 the encephalocmle or cerebral ventricle (Fig. 743, e?i. ca3., 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 



