228 ARTHUR WILLEY. 



9. Since, as will be shown below, the pharynx of Amphioxus 

 is not homologous, in the true sense of the word, with the 

 branchial sac of Ascidians, the adult position of the eudostyle 

 of Amphioxus cannot be homologous with that of the Ascidian 

 endostyle. The observations recorded in this paper, however, 

 prove unquestionably that the position of the endostyle in the 

 adult Amphioxus is secondary, and that in its origin it is, as 

 stated above, perfectly homologous with the endostyle of the 

 Ascidians. 



Further Homologies. 



It will have been inferred, from what has been stated above, 

 that the pharynx of Amphioxus, which is part of the trunk, is 

 not truly homologous with the branchial sac of Ascidians, which 

 is part of the prsechordal vesicle ; so that we have to face the 

 paradox that while a highly specialised organ of (speaking 

 generally) identical structure and relations, and extending the 

 whole length of the pharynx, is present in both cases, yet the 

 pharynx taken as a whole of the one is not homologous with 

 that of the other. This, no doubt, sounds at first like a 

 reductio ad absurdum of a morphological problem, but 

 the figures in the plates accompanying this paper, showing the 

 primary and secondary positions assumed by the endostyle of 

 Amphioxus, should assist in the realisation of this at first 

 startling paradox. 



The gill-slits or branchial stigmata of the Ascidians are 

 homologous with those of Amphioxus only in the sense that 

 they are distinct vertebrate structures, but they are not 

 homologous in their origin, position, or relations. There is 

 thus a sort of group homology existing between them. 



In Amphioxus the slits form metamerically in the segmented 

 region of the trunk. In Ascidians they form in front of the 

 (however imperfectly) segmented region, and, moreover, van 

 Beneden and Julin point out that they do not arise metameri- 

 cally one after the other, but irregularly ; for instance, the first- 

 formed slit will be the fourth of the series eventually, and so on. 



If these conclusions are as sound as they appear to be, it is 



