W. Z. BROOKS ON THE GENUS SALPA. 131 



evidence of comparative anatomy, and explain modern ontogeny as the 

 result of secondary modification. 



Comparative anatomy shows clearly that the atrium of Doliolum is 

 homologous with that of Pyrosoma and the Ascidians, and the onto- 

 genetic history in Doliolum does not present any great difficulty. 



I shall have to refer again further on to the nature of the evidence 

 from embryology, but I think all morphologists agree that when organs 

 or animals which are shown by their anatomy to be homologous, differ 

 in their ontogeny, we have good ground for expecting to find evidence 

 that the ontogeny has undergone secondary modification, and that very 

 considerable embryological diversity is quite compatible with close syste- 

 matic affinity. 



On the other hand, when two animals whose anatomy does not for- 

 bid comparison exhibit striking ontpgenetic resemblances, these must be 

 held to be evidence of phylogenetic relationship. 



The strongest evidence of the affinity of Salpa and Pyrosoma is of 

 this sort, and every student who has concerned himself with either the 

 embryology or the asexual multiplication of these animals has expressed 

 or implied his strong conviction of their relationship. 



My own studies have forced me to differ from Salensky most essen- 

 tially regarding the part which the follicle cells take in the construction 

 of the salpa embryo, but I fully agree with him (17), p. 84, that the 

 embryological phenomena prove that Salpa and Pyrosoma are closely 

 related. The embryological evidence of their affinity is so fundamental 

 that their whole history must be studied before its weight can be fully 

 appreciated. 



I have shown in the chapter on the significance of the Salpa embryo, 

 that the history of its development can be explained only on the 

 hypothesis that it exhibits, in a high degree, secondary complications 

 which are shown at a much more primitive stage by Pyrosoma, which 

 is, so far as our knowledge goes, the only animal which does give us 

 any material aid in the interpretation of the Salpa embryo. I have also 

 shown that there is some reason for believing that the egg of Salpa has 

 at some time in the past been furnished with a food-yolk, and has had a 

 meroblastic mode of development like Pyrosoma, although it may be 

 that these characteristics never became as highly evolved in the ances- 

 tors of Salpa as they are in Pyrosoma. 



We must not forget, however, that while there is no other animal 

 known to us with an embryology as much like that of Salpa as Pyrosoma, 

 the differences between the two are very considerable. 



