Animals are an extremely diverse group of organisms, with all of them sharing a number of characteristics. In general biological terms, animals are multicellular eukaryotes that are heterotrophic, ingesting and digesting food inside the body. Their cells lack the cell walls that provide support in the bodies of plants and fungi. The majority of animals have muscle systems and nervous systems, responsible for movement and rapid response to stimuli in their environment.

Most animals reproduce sexually; one reason for this versus asexual reproduction is variation, with each offspring being the product of both parents, giving them a better chance of survival. In most animal species, a large, nonmotile (movable) egg is fertilized by a small, flagellated sperm; this forms what is called a diploid zygote (or the fertilized egg). The transformation of the zygote into an animal’s specific form depends on special regulatory genes in the cells of the developing embryo. (For more about cells and reproduction, see the chapters “Cellular Basics” and “Animal Behavior and Reproduction.”)