The author has published this book to serve as a guide to the dissection and comparative study of invertebrate animals. While the comparative feature runs through all the dissections in the course each one is usually complete in itself and does not depend upon any others. The teacher us thus not compelled to adopt the entire series in order to have his course completed. In most of the dissections the directions as well as the order in which the different system of organs as taken as so arranged that the student can complete the study with a single specimen and therefore the accessity of upracting economy of materials is thus inculcated. The principal differences between the first and second editions of this work consist in the additions of several dissections in the second edition of the revision of the scheme of classifications in the Appendix.