PREFACE 



This book was made necessary by a radical change, several years 

 ago, in the introductory course in zoology at the University of Michi- 

 gan. When the dissection of animal types in the laboratory was re- 

 placed by exercises bearing on the larger questions and fundamental 

 principles of biology, a textbook treating almost exclusively of the 

 morphology of representatives of the phyla, classes and orders was 

 obviously of small use. In the absence of any text which appeared 

 to fit the new departure, a book was provided in temporary form by 

 the organizers of the course. Until now the temporary volume has 

 sufficed. 



Were a textbook based on principles useful only in a course whose 

 laboratory exercises were also based on principles, the authors would 

 not now feel impelled to make the volume available to others. They 

 have accepted, however, the judgment of other biologists who find 

 that even in courses in which types are dissected in the laboratory, 

 the textbook should deal largely with biological principles, and with 

 classification and the morphology of types only as these subjects fit 

 themselves into the principles of the science as a whole. The best 

 teachers have, in fact, long been presenting to their students much of 

 the material of this book in lectures and recitations. In the hope 

 that a textbook based on principles will be thought better than those 

 now commonly in use, even for courses which retain the dissection of 

 types in the laboratory, this volume is presented to the teachers of 

 zoology and biology. 



It is believed, furthermore, that the general reader will find in its 

 pages much that is of interest. Illustrations have been selected with a 

 view to making the book intelligible, not only to college students, but 

 to persons who are not pursuing laboratory work. The authors have 

 made no attempt to write in popular style for the benefit of such readers, 

 but have endeavored to make the exposition clear and in the main 

 specific. They prefer to believe that the general reader is willing to 

 labor for his acquisitions so long as his effort promises to yield com- 

 mensurate results. 



Advice and criticism have been freely asked and given in the prepara- 

 tion of many of the chapters. The authors desire to mention in par- 

 ticular Professors H. H. Bartlett and J. F. Shepard, Dr. L. V. Heilbrunn, 

 Dr. 0. M. Cope, Dr. P. 0. Okkelberg and Margaret B. Shull. 



A. Franklin Shull. 



February, WW. 



ix 



