This book is in-depth detailing of invertebrate phylum study for University students and other. It is designed to be read as a whole, to interest and memories the complicated terminology. The book chapters are made according to the phylum. Each phylum is described first with its general characters, classification, economic importance and selected examples are discussed in details. Each example is illustrated with morphological characters, nutrition, locomotion, and reproduction aspects. The diversity of the invertebrate is a major problem in writing about them in concise form. Any such attempt will result in enormous quantity of indigestible morphological and taxonomical information. We have tried to overcome this problem by being selective. We have selected only a few very commonly observed in various syllabuses related examples of each major group and limited description to points we considered interesting and important. I avoid including any phylum, which has fewer than a thousand known species, but I have described some small groups because knowledge of them improves our understanding of larger groups.