This book is comprised of important contributions from expert researchers around the world concerning the biology of animals from a variety of approaches. In particular are manuscripts that deal with cellular, biochemical, genetic, reproductive, and ecological themes including various manuscripts regarding invertebrate animals. Regarding this last animal class, the science provides excellent models to study a lot of biological processes that can explain the evolution and diversity of life on the earth’s surface. They are simple organisms, the studies of which can contribute to explanations of how the metabolic processes found in vertebrates and humans started and have been maintained. This book also provides results that demonstrate some effects and interactions among environmental conditions and drugs on the morphology and biochemical processes in cells, and it contains reviews concerning the interaction between pathogenic invertebrates and human diseases. The aim of these studies was to provide important results that are not commonly treated in traditional and experimental models within the current scientific scene.