SEVENTH EDITION 



College 

 Zoology 



ROBERT W. HEGNER 



Ph.D., Sc.D. 



LATE PROFESSOR OF PROTOZOOLOGY IN 



THE SCHOOL OF HYGIENE AND PUBLIC HEALTH 



OF JOHNS HOPKINS UNIVERSITY 



KARL A. STILES 



M.S., Ph.D. 



PROFESSOR AND HEAD, DEPARTMENT OF 

 ZOOLOGY, MICHIGAN STATE UNIVERSITY 



THE MACMILLAN COMPANY 

 NEW YORK 



