198 W. K. BROOKS. 



(1 7, pp. 456-462 and pp. 468-470 and 36-46), and as those who 

 wish can find in these papers an extended presentation of the com- 

 plicated and perplexing theory (?) of the mesoderm, I have attempted 

 to treat it very briefly. 



The literature shows that there is no direct evidence whatever of 

 the existence, at any time in the history of the tunicates, of a meta- 

 meric series of coelomic pouches, and the supposed necessity for 

 believing that such a series existed iu the primitive chordata is only 

 another aspect of the dogma that the metamerism of the vertebrates 

 must have been inherited from a primitive metameric ancestor. 



If, as I believe, the metamerism of vertebrates is secondary, the 

 metamerism of the mesoderm and body cavity may have resulted 

 from the duplication of a single pair of coelomic pouches similar to 

 those of echinoderm larvae, and it is quite conceivable that these 

 may have been acquired by the ancestors of the vertebrates after the 

 divergence of the tunicates. 



If, however, future research should show that there is a pair of 

 gut-pouches in the embryo of appendicularia, or should prove in 

 some other way that the structures which Salensky describes are 

 true enterocoels inherited ft'om an ancestral tunicate, such a dis- 

 covery, which is certainly among the possibilities, would be no 

 evidence that the primitive tunicate was the degenerated descendant 

 of an ancestor with metameric gut-pouches. 



At present, however, the evidence all tends to show that the ances- 

 tors of the tunicates had no such structure, and that the presence 

 of coelomic vesicles in pyrosoma is an adaptation to its peculiar mode 

 of development. 



