XIII 



PITYHIM OHORDATA 



53 



wide, backwavdly-iliivr-tcil cuds upcii intd the atriuin ; their 

 narrow anterior ends jimlial)!}' eouunnnieate with tlie cceloiiie. 

 There are also groups nf CDlnumar excretory cells on the Hoor of 

 the atrium. 



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

 organ, tliiL_iA*v*-r*fm or dorsal ncrvc-tuhc (Fig. 742, A, ri. ; B. neu., 

 Figs. 748, 744), contained within and completely filling a median 

 longitudinal neural 'canal which lies innnediateiy above the noto- 

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



Fir ;4il — Amphioxus lanceolatus. A, iiepliridium of the left side with part uf the 

 Willi of the pharynx. (From Willey, iifter Boveri.) 



it ends abruptly some distance behind the anterior end of the 

 notochord, while posteriorly it taper,s to a pomt o\er the hinder 

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

 cale (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 enccphaloccek or cerebral venlriele (Fig. 743, en. eo!., Fig. 747, 

 cv) and a litule behind this the dorsal fissure widens out above 

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



E 2 



