PREFACE vii 



serve to show which of the characters already met with are of 

 distinctive importance, and which special to the example itself. 

 In order to bring out this point more clearly, to furnish a connec- 

 tion between the account of the example and that of the class as a 

 whole, and to give some idea of the meaning of specific, generic, 

 and family characters, we have introduced, after the classification, 

 a paragraph giving the systematic position of the example, some- 

 times in more, sometimes in less detail. 



Following the table of classification with its brief definitions 

 comes the general account of the group. This is usually treated 

 according to the comparative method, the leading modifications of 

 the various parts and organs being described seriatim. In a few 

 cases this plan has been abandoned and the class described order 

 by order, but this is done only when the deviations from the type 

 are so considerable as to lead us to think the comparative method 

 unsuitable for beginners. On the other hand, when all the classes 

 of the phylum present a very uniform type of structure, the 

 phylum is studied comparatively as a whole. The description of 

 each group usually ends with some account of its ethology and 

 distribution, and with a discussion of its affinities and of the 

 mutual relationships of its various sub-divisions. 



We have done our best to make the space devoted to each group 

 proportional to its complexity and range of variation, and to 

 subdue the natural tendency to devote most attention to the more 

 recently investigated classes, or to those in which we ourselves 

 happen to be especially interested. A few lesser groups have been 

 put into small type, partly to economise space, partly because they 

 seem to us to be of minor importance to the beginner. 



Following out the plan of deferring the discussion of general 

 questions until the facts with which they are connected have been 

 brought forward, we have placed the sections on Distribution, on 

 the Philosophy of Zoology, and on the History of Zoology at the 

 end of the book. We have, however, placed a general account 

 of the structure and physiology of animals immediately after the 

 Introduction, and one on the Craniate Vertebrata before the 

 description of the classes of that division, but it will be obvious 

 that these deviations from the strictly inductive method were 

 inevitable in order to avoid much needless repetition. 



After a good deal of consideration we have decided to omit all 



