INTERNAL ANATOMY. 



77 



expands laterally and also in an upward direction, propor- 

 tionately reducing the extent of the ccelom as it does so 

 (Fig. 37 ; cf. also Fig. 26). At its posterior extremity the 

 atrial tube does not become closed in, but remains perma- 

 nently open as the atriopore. 

 It is a curious fact that the 

 fusion of the subatrial ridges 

 to enclose the atrial tube takes 

 place gradually from behind 

 forwards, so that for a long 

 time the latter has the form 

 of a canal open to the exterior 

 at both ends. The chief feat- 

 ures in the formation of the 

 atrium are shown diagrammat- 

 ically in Fig. 38, A, B, and C. 

 In Fig. 38 A the atrial tube 

 has not begun to be closed in, 

 but the two metapleural folds 

 are seen running side by side 

 for some distance. Anteriorly 

 the development of the right 



, . . , r , Fig. 18. Three plastic diagrams 



metapleur is in advance of that of lar ^ of Amphioxu ; from th * ven . 



of the left, and it is Seen tO tral aspect, illustrating the mode of 



enclosure of the atrial tube from be- 

 bend round tO the right Side hind forwards. The atrium is still 



of the body in correspondence S2VS?S ail* U^S 



with the asymmetry of the gill- closed in C. (After LANKESTER and 



WlLLEY.) 



slitS (vide infra). Having ar- p. s . Primary gill-slits. r.m. Right 



rived at the front end of the Praoialpit " Moutht 



pharynx, the right metapleur 



bends sharply inwards to the mid-ventral line and then 

 gradually dies out in front. In Fig. 38 B the subatrial 

 ridges have met and fused for a short distance behind the 



