PREFACE TO THE SECOND EDITION 



In preparing this revision, most of the chapters have been 

 rewritten and a considerable number of new illustrations have 

 been added. Sections on the. Protozoa and the Porifera have 

 been wholly recognised and the point of view and interpretations ^ 

 have been rather radically altered so as to bring the contents of 

 these chapters into accord with the recent discoveries and the 

 writings of the best recognized authorities. Six years of use 

 of the text with undergraduate classes have revealed the fact 

 that the original edition carried its material in too formidable 

 a framework of taxonomic organization. Without sacrificing 

 the standards of scientific accuracy, much material of general 

 biological interest has been introduced into the revised edition 

 to relieve the fault of overemphasis on morphology and taxonomy 

 in a general invertebrate text. The severity of the taxonomic 

 organization has been reduced further by the omission of specific 

 sections on the orders from most of the chapters. Pertinent 

 material formerly included under the ordinal headings has been 

 reorganized and placed in the general discussion of the phylum 

 or of the classes where it is less apt to become lost to the student 

 in the intricate maze of systematic relationships of the subgroups. 



To compensate for the removal of so much cf the formal 

 framework of classification from the body of the text, an outline 

 of classification has been added at the close of the discussion for 

 each phylum. Herein is given a terse characterization of all 

 classes, subclasses, and orders generally recognized for each 

 phylum. For review, for general extension of taxonomic horizon, 

 and for a comprehensive view of the interrelationships of the 

 various subdivisions of the phyla, these summaries give the 

 material an organization which is easily grasped and assimilated. 



Throughout the preparation of the revision, a number of 



advanced students in the University of Ilhnois have read and 



criticized sections of the manuscript and have generously assisted 



in the reading of proofs. tt i \t r^ 



^ ^ Harley J. Van Cleave. 



Urbana, III. 



April, 1931. 



