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 



