STUDIES ON THE PEOTOCHOBDATA. 341 



came to the conclusion that the '^primary branchial canals" 

 of the Ascidians were homologous with the anterior intestinal 

 diverticula of Amphioxus^ the two structures being to a greater 

 or less extent formed from hypoblastic evaginations, while in 

 Amphioxus the epiblastic element was entirely wanting. If 

 now the conception of " primary branchial canals" be abolished 

 this homology ipso facto falls to the ground. And it is 

 otherwise, as I have said above, rendered very improbable, 

 if not impossible, from the topographical relations of the 

 respective structures. 



Are, then, the atrial cavities of Ascidians, which subse^ 

 quently fuse together dorsally to form the single peribranchial 

 chamber of the adult, homologous with the atrial chamber of 

 Amphioxus ? Van Beueden and Julin, involved in their hypo- 

 thesis of the prechordal vesicle, denied that the atrial cavities, 

 together with the branchial stigmata of the Ascidians, were in 

 any way homologous with the atrial cavity and gill-slits of 

 Amphioxus. The question of the gill-slits has been already 

 discussed above. 



As for the atrial cavities, I could for a long time find no 

 ground for homologising them with the atrium of Amphioxus, 

 and in my short paper in the ' Proceedings of the Koyal 

 Society' (34), I spoke of them as being, at their first origin, 

 nothing but the ectodermic portions of the first pair of gill- 

 slits. By a subsequent renewed study, however, of their 

 origin in Clavelina I have become convinced that the atrial 

 cavities of Ascidians are structures sui generis, apart from 

 the stigmata which open into them. 



In fact, in Clavelina the atrial cavities appear at first 

 as longitudinal grooves, shallow anteriorly, and gradually- 

 deepening posteriorly up to the point where the actual perma- 

 nent involution occurs. In the series of transverse sections, 

 for instance, from which figs. 26 and 27 are taken, these 

 longitudinal grooves extend over at least eight sections cut at 

 a thickness of 7 fx. Five sections intervene between those 

 represented in figs. 26 and 27. 



In Amphioxus (28 and 32) the atrial chamber has a dis- 



VOL. XXXIV, PART III. — NEW SUK. Z 



