W. K. BEOOKS ON THE GENUS SALPA. 137 



their habit of life seems to be peculiarly favorable for the development 

 of the young upon or within the parent. 



While the large food-yolk of Pyrosoma, and the rudimentary con- 

 dition of its Cyathozooid, and the placenta and foetal life of Salpa, and 

 the differentiation of a set of Ascidiozoids to carry the sexual animals in 

 Doliolum and Anchinia, are obviously secondary, they do not prove that 

 the adults are any more modern than the Simple Ascidians, nor is the 

 reduction of the tail of the larva to a food reserve anything anomalous, 

 for in all ascidians it is used up as food after it has served its purpose, 

 and we might expect to find it directly devoted to this new use when its 

 locomotor function becomes unnecessary. 



It therefore seems to me that the reasons which Uljanin draws from 

 the facts of embryology for holding the view that the Simple Ascidians 

 are more primitive than the other Tunicates, and for regarding Pyrosoma 

 and Salpa as more divergent from the primitive type than Doliolum and 

 the Compound Ascidians, are perfectly consistent with a different inter- 

 pretation, and we find that Herdman's view is very different. 



Briefly summarized, his opinion is that Pyrosoma is not at all closely 

 related to Salpa and Doliolum, and that the two latter forms, with 

 Anchinia and Octacnemius, form a natural group, the Thaliacea, which 

 has been pelagic throughout its whole history, and has nothing in com- 

 mon with the Ascidians except its common descent from an ancestor 

 like Appendicularia, while he regards Pyrosoma as a fixed Compound 

 Ascidian which has secondarily become adapted to a swimming life. 



He says (p. 149), " It seems to me that the passage from Appendi- 

 cularia mossi through Anchinia rubra to Doliolum, and through the 

 ancestral Doliolidae to Salpa, is so natural and simple that it becomes 

 very improbable that the Thaliacea have ever been fixed forms. It is 

 extremely unlikely that they are, as Uljanin supposes, a group of Simple 

 Ascidians which, after being fixed, betook themselves again to a free 

 swimming mode of life." 



On page 124 he says that Appendicularia mossi "is perhaps the 

 nearest form known to the ancestral Tunicates, from which the two 

 great lines of degeneration (?) diverged, the one leading to the Doliolidse 

 and Salpida3, and the other to the Simple and Compound Ascidians." 



Herdman's introduction of Appendicularia mossi into his discussion 

 is unfortunate, and he also fails to give due weight to the evidence of the 

 affinity of Salpa and Pyrosoma which has been pointed out so often by 

 Huxley, Grobben, Salensky, and many others, and in view of all the facts 



