PREFACE. 



The Morphology of Invertebrate Animals may be treated 

 either from the standpoint of Comparative Anatomy or from 

 the zoological side, and either method of treatment has much 

 to recommend it. In my experience, however, the zoological 

 method has proved most satisfactory for the presentation of 

 the subject to students, inasmuch as it is necessarily the 

 method employed in the laboratory, and accordingly in the 

 present work that plan of presenting the facts of morphology 

 has been followed. A bare statement of the structural 

 peculiarities of the various groups, however, is simply collect- 

 ing the bricks and stones without the mortar necessary to 

 unite them together into a substantial edifice, and where the 

 opportunity has presented itself attention has been directed 

 to the comparative significance of various organs and to the 

 affinities of the various groups. 



A word is perhaps necessary in regard to the classification 

 adopted, which presents many radical changes from the schemes 

 usually employed. For the larger groups, following the ex- 

 ample of Glaus, the term type has been employed, and no less 

 than twelve of these types are adopted. This increased num- 

 ber has resulted from a division of two groups usually recog- 

 nized, namely, the Vermes and the Arthropoda. As regards the 

 former it has long been acknowledged to be a heterogeneous 

 collection, and its retention is to be regarded as a survival. 

 It is true that the forms assigned to it do present certain 

 phylogenetic affinities ; but if this is to be the reason for its 

 retention, then the Mollusca and Prosopygia (MoUuscoidea) 

 should also be assigned to it. It has seemed more satisfac- 

 tory to retain the Mollusca and Prosopygia as distinct groups, 

 and to divide the Vermes into several types, such as the 

 Platyhelminthes, Nemathelminthes, and Annelida, each of the 

 same rank as the Mollusca, and presenting approximately 

 similar degrees of affinity among themselves. 



