We have just finished the study of plants, the providers of the world. We will now examine the animals, "the eaters of the earth." To be an animal, an organism must be multicellular and heterotrophic. Most animals are invertebrates, lacking a backbone. Beyond these characteristics, various groups of animals distinguished themselves by certain evolutionary achievements. They have evolved characteristics, such as a body cavity and bilateral symmetry, that gives the animal advantages over the less evolved organisms. In this chapter, these and other developments are explained and evaluated. Although our species belongs to the most highly evolved animal group, it is not considered to be the most successful species. It is interesting to contemplate the meaning of success in the natural world.