Animals are a very diverse group of organisms, but all of them share a number of characteristics. Animals are multicellular eukaryotes that are heterotrophic, ingesting and digesting food inside the body. Animal cells lack the cell walls that provide support in the bodies of plants and fungi. Most animals have muscle systems and nervous systems responsible for movement and rapid response to stimuli in their environment. In addition, most animals reproduce sexually with the diploid stage dominating the life cycle. In most species a large, nonmotile egg is fertilized by a small, flagellated sperm, thus forming a diploid zygote. The transformation of a zygote to an animal of a specific form depends on the controlled expression in the developing embryo of special regulatory genes.