PREFACE X? 



becomes intelligible, and on the basis of laboratory study and 

 text should rest the lectures and other oral discussion. The 

 trouble with " principles " textbooks is that they have no founda- 

 tion in accounts of representative types of animals; and the 

 trouble with "phylum" textbooks is that they have no space 

 for an outhne of principles upon which the teacher can build his 

 own superstructure. 



The junior author is primarily responsible for the chapters on 

 Metabohsm, Irritabihty, the Cells of Vertebrates, and Genetics, 

 and has collaborated, by critical editing and by advising, through- 

 out the preparation of the remaining chapters as originally pro- 

 jected or written by the senior author. It is hoped that this 

 united effort has resulted in a better textbook than could othei-wise 

 have been produced. 



It is impossible, in a work of this nature, to acknowledge all 

 the sources from which assistance has been received. There are 

 the more remote influences, such as the authors have acknowledged 

 in the dedication, and those of former colleagues, including G. S. 

 Dodds and George Lefevre. The entire manuscript, as written 

 before the final revisions, was read critically by Professor E. A. 

 Andrews. Others who have read certain chapters are R. H. 

 Wolcott, E. A. Martin, F. L. Hisaw, and J. A. Dawson. Thanks 

 are due to the authors' colleagues in the department at the Uni- 

 versity of Missouri, who have collaborated in other ways, and to 

 George T. Khne and Helen Woelfel, biological artists. Other 

 acknowledgments of figures and of permission to use figures 

 appear in the legends. 



University of Missouri, W. C. CuRTIS 



Columbia, Missouri, M, J. GuTHfilE. 



March 1, 1927. 



