INTRODUCTION xiii 



of each organ and system of organs, to find out as fat- 

 as possible whether this resemblance was a case of 

 homology or analogy, and, if the former, to endeavour 

 to trace the causes which led to the transformation of 

 the carnivorous Annelid into the Crustacean. 



Shrewd conjectures have been made as to the 

 possible derivation of the Crustacea from Annelids, 

 but I am not aware that this point has ever before 

 been worked out in detail, and I should hardly have 

 ventured to undertake such a task had not my study 

 of Apus forced it upon me. 



My original intention x of preparing a comparative 

 anatomy of the Apodidae thus gave way before the 

 more ambitious attempt to use Apus as a key to 

 solve the hitherto unsolved problems as to the origin 

 of the Crustacea, and the true affinities between the 

 various groups. 



This resolution, however, was not formed at once. 

 The book is written in the order in which the subject 

 was worked out. 



The first part, which deduces Apus from a carnivo- 

 rous Annelid, was all I at first intended to publish. 

 Having never made a special study of Limulus nor 



1 Announced in a letter to Nature, reprinted in Appendix V. 



