In traditional classifications, any member of the kingdom Animalia, which comprises multicellular organisms that develop from embryos formed by the fusion of haploid eggs and sperm. Unable to manufacture their own food, they feed on other organisms or organic matter (see heterotrophic nutrition). Animals are therefore typically mobile (to search for food) and have evolved specialized sense organs for detecting changes in the environment; a nervous system coordinates information received by the sense organs and enables rapid responses to environmental stimuli. Animal cells lack the cellulose cell walls of plant cells. Molecular systematics now includes animals in the opisthokonts, a eukaryote assemblage that also contains the fungi. For a simplified classification of animals, see Appendix.