CHAPTER XIL 



THE TAXONOMY OF IXVERTEBRATED ANIMALS. 



The grouping of the various kinds of invertebrated ani- 

 mals which has been adopted in the preceding pages is to be 

 regarded merely as a temporary arrangement. Each chapter, 

 from the second to the tenth, is devoted to a series of forms, 

 the morphological relations of which are more or less obvi- 

 ous, while Ciiapter XL is reserved partly for such groups as 

 do not readily find a place in any of the series which precede 

 them ; and, partly, for such as have been established since 

 this work was commenced. 



Our knowledge of the anatomy, and especially of the 

 development, of the Invertebrata is increasing with such pro- 

 digious rapidity, that the views of Taxonomists in regard to 

 the proper manner of expressing that knowledge by classifi- 

 cation are undergoing, and, for some time to come, are likely 

 to undergo, incessant modifications. 



To the beginner, who is apt to make the mistake of look- 

 ing upon classification as the foundation and essence of mor- 

 phology, instead of what it really is, the superstructure and 

 outcome thereof, this state of things is distressing. Every 

 hand-book presents him with a different system of classifica- 

 tion, and he may, not unnaturall}^, despair of finding any 

 stability in a science, the most general results of which are 

 capable of being stated in such very diff'erent ways. If, how- 

 ever, the student will attend to the facts which constitute the 

 subject-matter of classifications, rather than to the modes of 

 generalizing them which are expressed in taxonomic systems, 

 he will find that, however apparently divergent these systems 

 may be, they have a great deal in common. 



It is possible to divide invertebrated animals into a certain 

 number of groups, each of which will be admitted by everj^ 

 morphologist to be in itself a perfectly natural assemblage. 

 That is to say. all the forms thus associated together will re- 



