INTERNAL ANA TOMIY. 



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 

 metapleur is in advance of that 

 of the left, and it is seen to 

 bend round to the right side 

 of the body in correspondence 

 with the asymmetry of the gill- 

 slits {vide infra). Having ar- 

 rived at the front end of the 

 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 

 ride;es have met and fused for a short distance behind the 



Fig. 38. — Three plastic diagrams 

 of larvni of Amphio.xus from tire ven- 

 tral aspect, illustrating the mode of 

 enclosure of the atrial tube from be- 

 hind forwards. The atrium is still 

 entirely unclosed in A ; partially 

 closed in B; and almost completely 

 closed in C. (After Lankester and 

 WiLLEY.) 



p.s. Primary gill-slits, r.vi. Right 

 metapleur. p.p, Praeoral pit. o. Mouth. 

 at. p. Atriopore. 



