138 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



of the case we must reject the first of his two lines of descent from 

 Appendicularia. 



The second line leading to Pyrosoma is perhaps pictured more 

 minutely than the facts warrant, but I think we can safely follow him : 

 1st, in his statement of the way in which the Ascidian line arose from 

 an ancestor like Appendicularia, and second, in his derivation of Pyro- 

 soma from an Ascidian-like ancestor which is not descended from the 

 Simple Ascidians. 



Finally I think we are fully justified in believing with G-robben, in 

 opposition to Uljanin and Herdman, that Salpa (including Octacnemius), 

 Doliolum (including Anchinia and Dolchinia), and Pyrosoma are more 

 closely related to each other than to any other known organism, but I 

 do not see how we can safely commit ourselves to a more minute 

 phylogeny. 



Gastrang, in a recent paper on the gill-slits of Tunicates (Proc. Eoyal 

 Soc. London, May 19, 1892, pp. 505-513), holds that the transverse stig- 

 mata of Pyrosoma are protostigmata; that they present conditions which 

 are embryonic or larval in the Ascidians, and that all the secondary stig- 

 mata in a transverse row, in the pharynx of an Ascidian, correspond to 

 or are homologous with a single one of the transverse protostigmata of 

 Pyrosoma. 



He therefore believes that the pharynx of Pyrosoma approaches 

 nearer to the ancestral type than that of the Ascidians. 



The literature of the subject is not all of it in harmony with his view, 

 but I see no valid ground for rejecting his conclusion, which is certainly 

 worthy of a provisional acceptance. Speculations upon the ancestry of 

 Pyrosoma must then conform to the conditions which it imposes, and 

 we cannot derive the pharynx of Pyrosoma from that of any fixed 

 Ascidian which we know. 



The facts seem to me to prove that Salpa and Pyrosoma are 

 descended from a fixed form, although Gastrang's studies seem to force 

 us to believe that this fixed form resembled Pyrosoma rather than the 

 Ascidians in the structure of its pharynx, and that it was in this respect 

 more primitive than any Ascidian which we know, although I see no 

 reason why it may not have been the parent of the fixed Ascidians, or 

 why it may not have resembled them in habit of life and in general 

 structure. 



In this discussion I have accepted without comment the opinion 

 which seems to be shared by all recent writers on the Tunicata, that 



