W. K. BROOKS ON THE GENUS SALPA. 55 



We therefore have only a slight basis for a phylogenetic history of 

 the modifications which the salpa embryo has undergone in reaching its 

 present form. 



We must remember, though, that it is possible to make instructive 

 and valuable comparisons even when they do not lead to exact or definite 

 phylogenetic conclusions, as when we compare adult echinoderms with 

 each other without committing ourselves to any view of their ancestral 

 relationship ; and I think we may give clearness and definiteness to our 

 conception of the salpa embryo by comparing it with the embryos of 

 other Tunicates, although we may not be able to mark out the path it 

 has followed in reaching its present structure. We cannot have a clear 

 notion of structure without comparison, although we may make note- 

 worthy progress without historical data. 



Even if we were totally without evidence as to the history of the 

 secondary modifications which the salpa embryo has undergone, their 

 morphological nature could be studied by comparing it with the embryos 

 of other Tunicates, and I shall show that we do have some evidence, 

 although it is true that this is scanty. 



The Embryology of Primitive Tunicates. 



The embryology of Pyrosoma, the nearest relative of Salpa, is com- 

 plicated by the early degeneration of the embryo, and by the early 

 appearance of asexual multiplication by buds. Both these peculiarities 

 are secondary, and they have been acquired, or at least very much 

 accented since Pyrosoma and Salpa diverged from their common 

 ancestor. We may therefore leave the four Ascidiozooids of the Pyro- 

 soma embryo out of consideration, and take the Cyathozooid as a basis 

 for comparison with the salpa embryo, but we must do away in imagi- 

 nation with the degeneracy of the Cyathozooid, and must picture it as 

 an embryo with the potency of an adult, like the embryo which becomes 

 a solitary salpa. 



We may also assume that, at some time in the past, these embryos, 

 like the embryo of Doliolum, passed through an Appendicularia-like 

 larval stage, corresponding to the tadpole larva of other Tunicates. Still 

 further back, we must believe that at some remote period the ancestral 

 embryo of all the Tunicates and of all other chordata passed through an 

 invaginated gastrula stage, formed by the total regular segmentation of 

 a holoblastic egg. We are also forced, by all the facts of embryology, to 



