vi PREFACE TO THE THIRD EDITION 



tion. A short chapter on environment and distribution has been 

 added. The final chapter on systematic zoology has been con- 

 siderably enlarged. A glossary has been included and also a list 

 of references for further reading. Some fifty additional illus- 

 trations have been inserted. 



The author is grateful to his colleagues, Dr. C. K. Weichert 

 and Dr. W. A. Dreyer, for helpful suggestions and criticisms, 

 and to his wife for invaluable aid in preparing the index. 



H. L. WlEMAN. 



University of Cincinnati, 

 May, 1938. 



