W. K. BROOKS ON THE GENUS SALPA. 135 



pinnata seems to have more features of resemblance to Pyrosoma than 

 any of the others, but I think this is all we can state with any confi- 

 dence. 



I am quite unable to satisfy myself as to the exact nature of their 

 relationship, or to formulate it either in definitions or in a genealogical 

 tree. 



SECTION 5. The Nature of the Relationship of Salpa, Doliolum, and 



Pyrosoma to the Ascidians. 



This subject has been discussed by both Uljanin and Herdman, but 

 there is little harmony in their results, although they agree that Pyro- 

 soma must be regarded as a descendant from the Compound Ascidians. 



Uljanin says that we are forced to believe that Doliolum has branched 

 off directly from the Simple Ascidians, since its '' ausserordentlich 

 grosse " relationship to them is proved by its embryology. His account 

 of its embryology is very peculiar, however, as he says that the archen- 

 teron of the gastrula disappears, and that the entire digestive system of 

 the adult is formed subsequently from an involution of the ectoderm at 

 the oral end of the larva. As nothing of this sort has ever been described 

 in the Simple Ascidians, the embryology of Doliolum can hardly be 

 stated to bear any " unusually great " resemblance to that of the Simple 

 Ascidians. 



Uljanin, basing his discussion of the relationships among the Tuni- 

 cates upon their chordate affinities, and upon the ancestral significance 

 of Appendicularia and the Ascidian tadpole, assumes, as his fundamental 

 principle, that those Tunicates which have most perfectly retained their 

 primitive or ancestral ontogeny, as this is exhibited by primitive chor- 

 data, must themselves be most primitive. Even if we admit this, we see 

 that the ontogeny of Doliolum by no means proves its primitive character, 

 for we are forced by his own account to believe that there has been very 

 great secondary modification in its very early stages of development. 



The principle cannot be accepted, however, as all embryologists 

 know. No one would hold that a starfish, in which the ontogeny has 

 been simplified by development in brood pouches, is more modern than 

 one with a free larval life. The modified ontogeny is more modern than 

 the unabridged ontogeny, of course, but it by no means follows that the 

 adult animal is also modern. 



As the ontogeny of the Simple Ascidians adheres more closely than 

 that of the Compound Ascidians to the primitive chordate type, as this 



