W. K. BEOOKS ON THE GENUS SALPA. 163 



reader to that chapter for my reasons for including appendicularia, the 

 copepods and the veiled medusae among the primitive pelagic animals. 



The Phytogeny of the Metazoa. 



The primitive pelagic fauna, before the influence of the bottom and 

 of the shore had been brought to bear upon it, consisted of small animals 

 of simple structure; but we are forced, by the facts of comparative 

 anatomy and embryology, to believe that a number of distinct types of 

 structure were found among them. 



Most of the great metazoic stems show by their embryology that 

 they run back to simple and minute pelagic ancestors, and that their 

 common meeting-point must be projected back to a still more remote 

 time, before the differentiation of their pelagic ancestors had been 

 effected. After we have traced each great line of metazoa as far back as 

 we can from the study of fossils and by the aid of comparative mor- 

 phology, we still find these lines distinctly laid down. The lower Cam- 

 brian Crustacea, for example, are as distinct from the lower Cambrian 

 echinoderms or pteropods or brachiopods or lamellibranchs, as they are 

 from those of the present day. The efforts of anatomists and embry- 

 ologists to reconstruct the primary phylogeny of the metazoa have so 

 far yielded few trustworthy results, and the results which are most 

 trustworthy are usually those which are the most indefinite. 



We are therefore forced to believe that the early steps in the estab- 

 lishment of the various types of metazoa were taken under conditions 

 which had some essential difference from those which have prevailed, 

 without any fundamental changes, from the time of the oldest fossil to 

 the present day ; and we are also forced to believe that most of the great 

 lines of descent were represented at some time in the remote past by 

 ancestors which, living a different sort of life, differed essentially in 

 structure as well as habits, from the representatives of the same types 

 which are known to us. Furthermore, embryology teaches that each 

 great group still bears internal evidence of descent from pelagic ances- 

 tors, and while the characteristics of these ancestors are in most cases 

 unknown, a few, like appendicularia, are still found alive. 



Our knowledge of the evolution of the metazoic types has certain 

 general features which are essentially the same for all, but each group 

 has also in its history much that is individual, and any general state- 

 ment requires so much qualification that the history of an illustrative 

 group is more instructive than a general summary. 



