24 THE VOYAGE OF H.M.S. CHALLENGER. 



buds from the position where they are formed to the dorsal sides of the parent 

 Ascidiozooids (see below, p. 33). 



Pyrosoma, although pelagic in its habits, is not at all closely related to tlie other 

 pelagic Tunicates — the Larvacea and the Thaliacea. Huxley at one time (18.51) pointed 

 out the similarity in some details of structure between Pyrosoma and Salvia ; but 

 that seems to have been more with the object of showing that the Tunicata could not 

 be divided into two great groups, the IMonoehitonida and the Dichitonida, than for the 

 purpose of demonstrating any close relationship between the two genera, since he at 

 the same time recognised the similarity in structure between Pyrosoma and Botryllus. 

 Savigny also had long liefore pointed out the resemblance of Pyrosoma to the 

 Compound Ascidians. In the second part of this Eeport ^ I insisted strongly on this 

 as being the true relationship of the Ascidise Salpiformes, and I showed how the new 

 family Ccelocormidge, then created for the remarkable Ccelocormus huxleyi, formed a 

 link connecting tlie aberrant Pyrosoma with such normal Compound Ascidians as the 

 Distomidse. 



Uljanin," in discussing the relationship of Pyrosoma, has connected it with the 

 Compound Ascidians by means of Distaplia, his reason for so doing being apparently 

 that gemmation is effected in DistapUa by means of a ventral stolon, as is the case in 

 Pyrosoma. It is possible, however, that too much has been made of the peculiarities 

 of Distaiylia. It seems to me to be an ordinary typical Compound Ascidian, belonging 

 to the family Distomidae, and having no closer relationship to Pyrosoona, or to the 

 Thaliacea, than is found in many other Ascidise Compositse. As to the ventral stolon, 

 many Compound Ascidians produce buds upon a stolon or outgrowth which is ventral 

 in origin. The vascular stolon of the Clavelinidis, and the post-abdomen of the Poly- 

 clinidse, from both of which buds are formed, are in part prolongations from, or contain, 

 the heart, which is admitted to be a ventral vessel. Even the remarkable process of 

 gemmation seen in the Didemnidse and Diplosomidte, where the body of the new Ascidio- 

 zooid appears at first to be formed by the union of two distinct outgrowths from the 

 body of the parent, is merely a modification of the same process, since the two bud rudi- 

 ments are simply the divisions of a single ventral outgrowth or stolon. Consequently 

 in most, if not in all. Compound Ascidians, gemmation is effected by means of a more 

 or less modified ventral stolon, and the probability is, that the process is the same in 

 Ccelocormus. In other respects I regard Ccelocormus as being more nearly related in 

 structure to Pyrosoma than is any other Compound Ascidian. The remarkable life- 

 history of Pyrosoma, the formation of the Cyathozooid, from which the first four 

 Ascidiozooids of the colony are formed, at first seems to be quite peculiar to this genus, 

 and unlike anything seen in the Compound Ascidians ; but after all it may be regarded 

 as a form of embryonic blastogenesis, — a process which is already known to occur in the 

 1 Zool. Chall. Exp., part xxxviii. 1886. 2 Fauna u. Flora d. Golfes v. Neapel, Monogr. x. p. 123. 



