88 The Atrial Chamber of Amphioxus. [May 22, 



FIG. 9. 



-my: 



Transverse section through an adult Amphioxus. The atrium has grown up so 

 as to divide the primitive splanchnocoel into two portions an inner or splanchnic, 

 and an outer or parietal, portion (sp' . and sp."). 



sp. The portion of the primitive splanchnocoel which is not so affected by 



the atrium, and which persists as the dorsal coelom. 

 sp". The parietal part of the splanchnocoel. 

 sp"'. Its expansion as perigonadial coelom. 

 f.r. Fin-ray. 

 Other letters as in figs. 6 and 7. 



(The above figures 6 9 are all somewhat diagrammatic.) 



tube widens and pushes itself right and left, so as to encroach on the 

 space hitherto occupied by the coelom, and finally it extends so far 

 dorsalwards as nearly to surround the alimentary canal (see figs. 8 

 and 9). 



The evidence of this history, in the form of careful drawings of 

 consecutive sections from snout to atriopore, at various stages in the 

 closure of the atrium, I purpose to publish shortly in the ' Quarterly 

 Journal of Microscopical Science.' For the present I am anxious to point 

 out, firstly, that this mode of formation of the atrium as a narrow groove, 

 which closes and sinks (as it were) into the body of the Amphioxus, 



