STUDIES ON THE PROTOCHORDATA. 323 



ancestor of the Protochordata could exist, and, moreover, in a 

 free-swimming animal. 



In Sagitta, again, we have a pair of head-cavities which are 

 very possibly homologous in a certain way with the head- 

 cavities of Amphioxus, but which do not occur in such a way 

 as to produce a praeoral lobe, and therefore do not prevent the 

 mouth from holding its anterior terminal position. The 

 prseoral lobe or proboscis of Balanoglossus, as well as its 

 homologue which I claim to have identified in the Ascidian 

 larva, represents a pair of head-cavities analogous to those that 

 occur in Sagitta — although I do not wish to assert a genetic 

 relationship between the former and the latter — which, how- 

 ever, have acquired such a mode of development as to produce 

 by their fusion a large median lobe in front of the mouth. 

 The prseoral lobe, however, while standing in the way of a 

 terminal mouth, does not, as I have said above, determine 

 whether the mouth shall be dorsal or ventral. That is 

 dependent on other circumstances, such as the mouth coming 

 into important relations to the central nervous system. 



In Balanoglossus and in the Ascidians the two head-cavities 

 do not appear as such distinctly paired structures in the 

 ontogeny of the individual as they do in Amphioxus. And in 

 the latter case, as is well known, they do not fuse together, but 

 remain distinct, one of them undergoing hypertrophy and 

 giving rise to the prseoral body- cavity and the other to the 

 praeoral pit. 



This hypertrophy of the head-cavities in the forerunners of 

 the Protochordata necessitated a change in the position of the 

 mouth, and a removal from its primitive situation at the 

 anterior terminal extremity of the body. Along the line of 

 descent which led to Balanoglossus the mouth migrated along 

 the ventral side of the body, and along the line of descent 

 that led to Amphioxus and the Ascidians the mouth passed 

 along the dorsal side, but in all cases the identity of the 

 head-cavities and of the mouth remained unaffected. 



I have thus shown a possible means of explaining the 

 discrepancy between the primitive position of the mouth in the 



