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 Student who would acquire a satisfactory knowledge of the principles of Zoology is recommended to commence by learning the elementary principles of General Biology; and having mastered these he should then study the groups of the Invertebrates as here detailed, coupling his study with a practical examination of such common types as are easily to be obtained. A jellyfish, or a hydra, an earthworm, an oyster, a snail, a cockroach and a lobster, are forms everywhere procurable, and, if examined, will give the student a good general idea of the structure of Invertebrate Animals. It must be borne in mind that without some such practical study, no amount of reading will suffice to convey accurate and adequate ideas of animal organization. [This content was specially revised for America by A. S. Packard, Jr.]