324 AETHUR WILLEY. 



Ascidian tadpole and the larva of Amphioxus and in Balano- 

 glossuSj which may at least serve as a working hypothesis. 

 My main object has been to point out, that the fact that the 

 mouth lies dorsally or ventrally has nothing to do with the 

 homology of the prseoral body-cavity in all the forms in 

 question. The homology between the prseoral coelom of 

 Balanoglossus and the head-cavities of Amphioxus was urged 

 very strongly by Bate son, but it is most important to 

 remember, as has been repeatedly pointed out, that the mouth 

 of Amphioxus, although ventral in the adult, has, as I think 

 beyond a shadow of a doubt, descended from a primitively 

 dorsal position in the neighbourhood of the neuropore. In 

 other words, the mouth in Amphioxus originally possessed the 

 same topographical relations as it does in the Ascidian tadpole; 

 and if the prseoral body-cavity of Amphioxus is homologous 

 ■with the corresponding structure in Balanoglossus, so is the 

 prseoral body-cavity of the Ascidian tadpole. The above 

 considerations all tend to establish the accuracy of my identifi- 

 cation of the latter structure. 



The diagrams on PI. 12 will place the whole question here 

 discussed in the clearest possible light. From these diagrams 

 it will be at once seen that the mouth of the larva of Amphioxus 

 occupies an intermediate position between that which it holds 

 in the Ascidian larva and in Balanoglossus, but I am very far 

 from meaning to suggest that phylogenetically it represents an 

 intermediate stage between these two extremes. On the con- 

 trary, it certainly does not. There is no evidence whatever to 

 suppose that the mouth of Balanoglossus has migrated from a 

 dorsal to a ventral position. As has been said above, it is 

 probable that both the mouth of the Ascidian and that of 

 Balanoglossus have attained their present situation from an 

 ancestral terminal position. 



Errata. 



In No. I of these " Studies on the Protochordata " one or 

 two trifling lapsus calami, which I had not the opportunity 

 of correcting in the proof, crept into the text. On p. 348^ re- 



