W. K. BROOKS ON THE GENUS SALPA. 



127 



Uljanin believes (p. 123) that we have in Salpa, Doliolum, and Pyro- 

 soma, representatives of three distinct and independent lines of descent 

 from the Simple Ascidians, from which Salpa and Doliolum are directly 

 although independently descended, while Pyrosoma is a modified Com- 

 pound Ascidian, and is, together with all the Compound Ascidians, 

 descended from the Simple Ascidians. 



Uljanin gives a genealogical tree of the Tunicates, which, so far as 

 the forms which we are now examining are concerned, is like this- 

 Pyrosoma Doliolum Salpa 



Compound Ascidians 



Simple Ascidians 



Appendicularia 



Grobben and Uljanin agree in the opinion that Salpa, Doliolum, and 

 Pyrosoma are all of them descended from Ascidians, but here the agree- 

 ment ends, while Herdman, p. 124, rejects this very point, and holds that 

 Salpa and Doliolum are descended directly from Appendicularia, and 

 that they have nothing else in common with the Ascidians. He believes 

 that Salpa and Doliolum are closely related, and that they form together 

 a natural group, while Pyrosoma belongs to another line entirely, and is 

 descended from a Compound Ascidian ancestor. 



While these views do not exhaust all the permutations and combina- 

 tions of the three factors, they are enough to show the absence of a 

 common ground for comparison, although it should not be difficult to 

 place the subject upon a sound and permanent basis, as the forms 

 which are to be considered are so very few and our knowledge of them 

 so complete. 



