PREFACE 
THE plan of this course is to study each of the larger groups 
of invertebrate animals, so far as possible, as a whole, instead 
of detached types of different groups taken more or less at 
random, as is usually done. The attention is directed con- 
stantly to the main structural features which characterize the 
entire group under consideration. The effort is thus made to 
teach relationships, and to make the study truly comparative. 
In order that the systematic position of the animals examined 
and their larger affinities may be easily kept in mind, a synopsis 
of the animal kingdom expressing the relationships of the 
various groups has been added in an appendix. 
The course begins with arthropods, because the natural 
succession of forms from the lowest to the highest is more 
apparent in them than in any other group of invertebrates, 
and it is, consequently, easier for a beginner, by studying 
them, to learn to appreciate the real significance of the blood- 
relationship of animals. Arthropods are also perhaps the most 
convenient animals with which to teach the fundamental prin- 
ciples of invertebrate morphology. Whether, however, the 
student begins his course with insects or with crustaceans, 
and whether the first insect taken up is the wasp or the 
grasshopper, will be matters for the decision of the teacher. 
The course has been so arranged that any of these methods of 
beginning may be adopted. 
While the comparative feature runs through all the dissections 
in the course, each one is usually complete in itself and does 
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