This Series is designed to meet the requirement of brief textbooks both for schools and for adult readers who wish to review or expand their knowledge. The grade of the books is intermediate between the so-called "primers" and the larger works professing to present quite detailed views of the respective subjects. Although the treatment of the subject is strictly elementary, the fundamental facts are stated and discussed with the fulness needed to place their scientific significance in a clear light, and to show the relation in which they stand to the general conclusions of science. This book contains two volumes: (1) Zoology of the Invertebrate Animals; and (2) Zoology of the Vertebrate Animals. [This content was specially revised for America by A. S. Packard, Jr.]