PREFACE TO SECOND EDITION 



The present edition represents a complete editing of the previous 

 edition with some revision and addition of material at certain 

 points. The general form and content of the book have been re- 

 tained. A number of condensed and pertinent tables have been 

 added to the chapter on Physiology, and a new section dealing 

 with the snail has been included in the chapter on mollusca. The 

 author makes acknowledgment here for many valuable suggestions 

 which came from Dr. C. J. Goodnight of Purdue University. Also, 

 appreciation is expressed to Betty Ann Potter for her valuable 

 assistance with the index of this book. 



George E. Potter. 



College Station, Texas. 



