PREFACE. 
Tuts book is designed to be used quite as much in the la- 
boratory or with specimens in hand, as in the class-room. If 
Zoology is to be studied as a mental discipline, or even if the 
student desires simply to get at a genuine knowledge, at first 
hand, of the structure of the leading types of animal life, 
he must examine living animals, watch their movements and 
habits, and finally dissect them, as well as study their mode 
of growth before and after leaving the egg or the parent, as. 
the case may be. But the young student in a few weeks’ 
study in the laboratory cannot learn all the principles of the 
science. Hence, he needs a teacher, a guide, or at least a 
manual of instruction. This work is an expansion of a 
course of lectures for college students, but has been pre- 
pared to suit the wants of the general reader who would ob- 
tain some idea of the principles of the science as generally 
accepted by advanced zoologists, in order that he may under- 
stand the philosophical discussions and writings relating to 
modern doctrines of biology, especially the law of evolution 
and the relations between animals and their surroundings. 
The book has been prepared, so far as possible, on the in- 
ductive method. The student is presented first with the 
facts; is led to a thorough study of a few typical forms, 
taught to compare these with others, and finally led to the 
principles or inductions growing out of the facts. He has 
not been assailed with a number of definitions or diagnoses 
applicable to the entire group to which the type may belong 
before he has learned something about the animals typical 
