PREFACE TO THE THIRD EDITION. 



THE short interval which has elapsed since the preceding preface 

 was written affords additional evidence of the increasing useful- 

 ness of this work. Encouraged by such success, the. Author, in 

 revising the present edition, has supplied sundry omissions, and 

 added such new observations as the onward progress of ana- 

 tomical science seemed to require. Important alterations in the 

 arrangement of the Animal Series have likewise been introduced, 

 among which may be pointed out the complete separation of the 

 PROTOZOA from the CILIATED INFUSORIA, the introduction and 

 redistribution of the class HELMINTHOZOA, the transference of 

 the classes ROTIFERA and CIRRHOPODA into close proximity with 

 the CRUSTACEA, to which they are related in many particulars of 

 their economy, and the establishment of the POLYZOA as legiti- 

 mate members of the MOLLUSCOUS division of Creation. As a 

 general rule, however, the Author has been careful to avoid un- 

 necessary changes in zoological classification, from a conviction 

 that they are rather calculated to embarrass than to facilitate the 

 progress of the student of Comparative Anatomy. 



