ANIMAL

any member of the kingdom Animalia, a group of multicellular organisms that are thought to have evolved independently from the unicellular eukaryotes. Animals differ from members of the two other multicellular kingdoms, the plants (Plantae) and the fungi (Mycota), in fundamental variations in morphology and physiology. At the cellular level, the most obvious distinction between animals and plants is that the animal cell wall is either absent or composed of a nitrogenous material, whereas the plant cell wall is composed of a carbohydrate materialcellulose. In general, animals are distinguished from plants and fungi by the capacity for spontaneous movement and rapid motor responses to stimuli. The mobility afforded by muscles in turn requires much more elaborate systems of sensory reception and internal communication and greater levels of general complexity than are found in plants or fungi. Animals account for about three-quarters of the living species on Earth; this diversity reflects the flexibility in feeding, defense, and reproduction that mobility gives them. The animal and the plant both require food; but, although both animals and plants take their water and inorganic salts directly as such, the animal cell can absorb its carbohydrate and protein food only in the form of complex organic substances; it is dependent, in fact, on the preexistence of these organic substances, themselves the products of living matter, and in this respect the animal is essentially a parasite on existing animal and plant life. In general, an animal is a living organism that is incapable of synthesizing carbohydrates and proteins from inorganic or simple organic substances but must ingest them in complex form as food. (kingdom Animalia), any of a group of multicellular eukaryotic organisms (i.e., as distinct from bacteria, their deoxyribonucleic acid, or DNA, is contained in a membrane-bound nucleus). They are thought to have evolved independently from the unicellular eukaryotes. Animals differ from members of the two other kingdoms of multicellular eukaryotes, the plants (Plantae) and the fungi (Mycota), in fundamental variations in morphology and physiology. This is largely because animals have developed muscles and hence mobility, a characteristic that has stimulated the further development of tissues and organ systems. Animals dominate human conceptions of life on Earth not simply by their size, abundance, and sheer diversity but also by their mobility, a trait that humans share. So integral is movement to the conception of animals that sponges, which lack muscle tissues, were long considered to be plants. Only after their small movements were noticed in 1765 did the animal nature of sponges slowly come to be recognized. In size animals are outdone on land by plants, among whose foliage they may often hide. In contrast, the photosynthetic algae, which feed the open oceans, are usually too small to be seen, but marine animals range to the size of whales. Diversity of form, in contrast to size, only impinges peripherally on human awareness of life and thus is less noticed. Nevertheless, animals represent three-quarters or more of the species on Earth, a diversity that reflects the flexibility in feeding, defense, and reproduction which mobility gives them. Animals follow virtually every known mode of living that has been described for the creatures of Earth. Animals move in pursuit of food, mates, or refuge from predators, and this movement attracts attention and interest, particularly as it becomes apparent that the behaviour of some creatures is not so very different from human behaviour. Other than out of simple curiosity, humans study animals to learn about themselves, who are a very recent product of the evolution of animals. Additional reading Technical descriptions of almost all taxa to the family level are presented in Sybil P. Parker (ed.), Synopsis and Classification of Living Organisms, 2 vol. (1982). The most detailed survey of animals is found in Pierre P. Grass (ed.), Trait de zoologie: anatomie, systmatique, biologie (1948 ), an ongoing multivolume work, published in parts. Libbie H. Hyman, The Invertebrates, 6 vol. (194067), is a classic, unfortunately incomplete, but careful, fairly detailed, and still mostly accurate.A survey of animal diversity is found in C. Barry Cox and Peter D. Moore, Biogeography, an Ecological and Evolutionary Approach, 4th ed. (1985). Developments in the preservation of biological diversity are discussed in Edward O. Wilson and Frances M. Peter (eds.), Biodiversity (1988). Laws of physics that directly influence the size and shape of living organisms are addressed in Steven Vogel, Life's Devices: The Physical World of Animals and Plants (1988); and R. McNeill Alexander, Animal Mechanics, 2nd ed. (1983). Knut Schmidt-Nielsen, Scaling: Why Is Animal Size So Important? (1984), discusses the significance of animal body size.For discussion of form and function, see Alfred Sherwood Romer and Thomas S. Parsons, The Vertebrate Body, 6th ed. (1986), a classic text; and E.J.W. Barrington, Invertebrate Structure and Function, 2nd ed. (1979), a survey by system. Robert D. Barnes, Invertebrate Zoology, 5th ed. (1987) and Richard C. Brusca and Gary J. Brusca, Invertebrates (1990), are surveys by group. W.N. Beklemishev, Principles of Comparative Anatomy of Invertebrates, 2 vol. (1969; originally published in Russian, 1944; 3rd Russian ed. 1964), provides good coverage of invertebrate development. Research-level reviews of reproduction in many groups are available in Arthur C. Giese and John S. Pearse (eds.), Reproduction of Marine Invertebrates, vol. 15 (197479), with a concluding synthesis provided in vol. 9 (1987) and intermediate volumes scheduled for publication in the 1990s. See also B.I. Balinsky, An Introduction to Embryology, 5th ed. (1981), a standard text that focuses on vertebrates only. A review of research on energy metabolism in most major animal groups is offered in T.J. Pandian and F. John Vernberg (eds.), Animal Energetics, 2 vol. (1987). Marcel Florkin and Bradley T. Scheer (eds.), Chemical Zoology, 11 vol. (196779), surveys the chemicals important to animal structure, physiology, and behaviour, organized by taxonomic group. Knut Schmidt-Nielsen, Animal Physiology: Adaptation and Environment, 4th ed. (1990), is a comprehensive introduction.For more detailed treatment of the fauna and ecology of major terrestrial and aquatic ecosystems, see the multivolume series Ecosystems of the World, published by Elsevier Scientific since 1977. F. Harvey Pough, John B. Heiser, and William N. McFarland, Vertebrate Life , 3rd ed. (1989), presents a broad overview from paleontology to animal behaviour. Robert E. Ricklefs, Ecology, 3rd ed. (1990), is an authoritative work. Thomas C. Cheng, General Parasitology, 2nd ed. (1986), provides a detailed introduction to the subject.For explorations of evolution and paleontology, see Douglas J. Futuyuma, Evolutionary Biology, 2nd ed. (1986), a general introduction; Martin F. Glaessner, The Dawn of Animal Life: A Biohistorical Study (1984), a useful discussion of the first recorded radiation of animals; Rudolf A. Raff and Thomas C. Kaufman, Embryos, Genes, and Evolution: The Developmental-Genetic Basis of Evolutionary Change (1983), a causal rather than descriptive approach to embryology in all animals; and Robert L. Carroll, Vertebrate Paleontology and Evolution (1988), a standard survey. Raymond C. Moore (ed.), Treatise on Invertebrate Paleontology (1953 ), a basic source of information, with descriptions and illustrations to generic level, is an ongoing multivolume work published in parts; newly revised editions of some parts began to appear in 1970, ed. by Curt Teichert. Paleontological topics are thoroughly reviewed in Richard S. Boardman (ed.), Fossil Invertebrates (1987); and Steven M. Stanley, Extinction (1987). On the origin and diversification of hormones, see E.J.W. Barrington (ed.), Hormones and Evolution, 2 vol. (1979). Ernst Mayr, Populations, Species, and Evolution (1970), is a revised abridgement of the author's classic book on speciation and other relevant topics. Virginia C. Maiorana Leigh M. Van Valen