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animal, (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.

The animal kingdom
Animals evolved from unicellular eukaryotes. The presence of a nuclear membrane in eukaryotes permits separation of the two phases of protein synthesis: transcription (copying) of deoxyribonucleic acid (DNA) in the nucleus and translation (decoding) of the message into protein in the cytoplasm. Compared to the structure of the bacterial cell, this gives greater control over which proteins are produced. Such control permits specialization of cells, each with identical DNA but with the ability to control finely which genes successfully send copies into the cytoplasm. Tissues and organs can thus evolve. The semirigid cell walls found in plants and fungi, which constrain the shape and hence the diversity of possible cell types, are absent in animals. If they were present, nerve and muscle cells, the focal point of animal mobility, would not be possible.

= A definition of animals = A characteristic of members of the animal kingdom is the presence of muscles and the mobility they afford. Mobility is an important influence on how an organism obtains nutrients for growth and reproduction. Animals typically move, in one way or another, to feed on other living organisms, but some consume dead organic matter or even photosynthesize by housing symbiotic algae. The type of nutrition is not as decisive as the type of mobility in distinguishing animals from the other two multicellular kingdoms. Some plants and fungi prey on animals by using movements based on changing turgor pressure in key cells, as compared with the myofilament-based mobility seen in animals. Mobility requires the development of vastly more elaborate senses and internal communication than are found in plants or fungi. It also requires a different mode of growth: animals increase in size mostly by expanding all parts of the body, whereas plants and fungi mostly extend their terminal edges.

All phyla of the animal kingdom, including sponges, possess collagen, a triple helix of protein that binds cells into tissues. The walled cells of plants and fungi are held together by other molecules, such as pectin. Because collagen is not found among unicellular eukaryotes, even those forming colonies, it is one of the indications that animals arose once from a common unicellular ancestor.

The muscles that distinguish animals from plants or fungi are specializations of the actin and myosin microfilaments common to all eukaryotic cells. Ancestral sponges, in fact, are in some ways not much more complex than aggregations of protozoans that feed in much the same way. Although the sensory and nervous system of animals is also made of modified cells of a type lacking in plants and fungi, the basic mechanism of communication is but a specialization of a chemical system that is found in protists, plants, and fungi. The lines that divide an evolutionary continuum are rarely sharp.

Mobility constrains an animal to maintain more or less the same shape throughout its active life. With growth, each organ system tends to increase roughly proportionately. In contrast, plants and fungi grow by extension of their outer surfaces, and thus their shape is ever changing. This basic difference in growth patterns has some interesting consequences. For example, animals can rarely sacrifice parts of their bodies to satisfy the appetites of predators (tails and limbs are occasionally exceptions), whereas plants and fungi do so almost universally.

History of classification
Except perhaps for the possession of collagen, the criteria used above to distinguish animals from other forms of life are not absolute. The first catalogs of animal diversity were based on overall form and similarity. Aristotle and other early biologists regarded all organisms as part of a great chain, divisions of which were more or less arbitrary. The 18th-century Swedish botanist Carolus Linnaeus divided all animals into six classes: Mammalia, Aves, Amphibia (including reptiles), Pisces, Insecta (Arthropoda), and Vermes (other invertebrates). In the early 1800s the French zoologist Georges Cuvier recognized that vertebrates were substantially different from invertebrates, and he divided most animals on the basis of form and function into four branches: vertebrates, arthropods (articulates), mollusks, and radiates (animals with radial symmetry). Cuvier’s divisions formed the basis for all subsequent classifications.

Just after Cuvier’s classification, the French naturalist Étienne Geoffroy Saint-Hilaire outlined the importance of homologous structures. Homology is correspondence between features caused by continuity of information. Thus, a bird’s wing is homologous to a bat’s wing insofar as both are forelimbs, but they are not homologous as wings. Homologous structures need not resemble each other; for example, the three bones in the middle ear of humans are homologous to three bones in the jaw apparatus in fishes because the genetic and developmental information controlling them has been continuous through evolutionary change.

Before evolution was generally accepted, homologies among different animals, when they were recognized at all, were regarded as aspects of God’s pattern. Evolution provided a testable explanation for homologies. By carefully tracing selected homologies, it has been possible to show that previously proposed classifications established inappropriate relationships based solely on form or function, or both; for example, the radial symmetry of starfishes is not homologous to that of coelenterates (such as jellyfish).

Protozoans were once considered to be animals because they move and do not photosynthesize. Closer study has shown, though, that their movement is by means of nonmuscular structures (cilia, flagella, or pseudopods) and that photosynthesis in them has often been lost and gained. Protozoans do not, therefore, form a natural group but with algae form a eukaryotic kingdom separate from plants and animals, called Protista.

Like plants and animals, fungi arose from protists and are now accorded a kingdom of their own.

= Animal diversity = The diverse appearance of animals is mostly superficial; the bewildering variety of known forms, some truly bizarre, can be assorted among a mere half-dozen basic body plans. These plans are established during the embryonic stages of development and limit the size and complexity of the animals. Symmetry, number and relative development of tissue layers, presence and nature of body cavities, and several aspects of early development define these fundamental modes of organization.

Parazoa: a cellular level of organization
Although the two phyla in this subkingdom, Porifera (sponges) and Placozoa, lack clearly defined tissues and organs, their cells specialize and integrate their activities. Their simplicity has been adaptive, and sponges have remained important in benthic marine habitats since their origin. The sessile, filter-feeding way of life shown by sponges has favoured a body plan of radial symmetry, although some members have become asymmetrical. The shape of the creeping, flattened placozoans is irregular and changeable.

Radiata: a tissue level of organization
The two coelenterate phyla (Cnidaria and Ctenophora) advanced in complexity beyond the parazoans by developing incipient tissues—groups of cells that are integrally coordinated in the performance of a certain function. For example, coelenterates have well-defined nerve nets, and their contractile fibres, although only specialized parts of more generalized cells, are organized into discrete muscle units. Because discrete cells of different types do not carry out the internal functions of the animals, coelenterates are considered to be organized at only a tissue level.

The integration of cells into tissues, particularly those of nerve and muscle, permits a significantly larger individual body size than is possible with other modes of body movement. Flagella and cilia become ineffective at rather small size, and amoeboid movement is limited to the size a single cell can attain. Muscles contract by a cellular mechanism basically like that used in amoeboid locomotion—interaction of actin and myosin filaments. Through coordinated contraction of many cells, movement of large individuals becomes possible.

Coelenterates, like parazoans, have only two body layers, an inner endoderm primarily for feeding and an outer ectoderm for protection. Between the endoderm and the ectoderm of coelenterates is the mesoglea, a gelatinous mass that contains connective fibres of collagen and usually some cells. Both layers contain muscle fibres and a two-dimensional web of nerve cells at the base; the endoderm surrounds a central cavity, which ranges from simple to complex in shape and serves as a gut, circulatory system, and sometimes even a skeleton. The cavity is also used for gamete dispersal and waste elimination.

Cleavage of a fertilized egg produces a hollow sphere of flagellated cells (the blastula). Invagination of cells at one or both poles creates a mouthless, solid gastrula; the gastrula is called the planula larva in species in which this stage of development is free-living. The inner, endoderm cells subsequently differentiate to form the lining of the central cavity. The mouth forms once the planula larva has settled. Although the details of early development are different for parazoans and coelenterates, most share a stage in which external flagellated cells invaginate to form the inner layer, which lines the cavity, of these diploblastic (two-layered) animals. This is characteristic of invagination during the development of all animals.

All coelenterates are more or less radially symmetrical. A radial form is equally advantageous for filtering, predatory, or photosynthetic modes of feeding. Tentacles around the circumference can intercept food in all directions.

Bilateria: an organ level of organization
All animals except those in the four phyla mentioned above have bilaterally symmetrical ancestors and contain three body layers (triploblastic) with coalition of tissues into organs. The body plans that are generally recognized are acoelomate, pseudocoelomate, and coelomate.

Acoelomates have no internal fluid-filled body cavity (coelom). Pseudocoelomates have a cavity between the inner (endoderm) and the middle (mesoderm) body layers. Coelomates have a cavity within the mesoderm, which can show one of two types of development: schizocoelous or enterocoelic. Most protostomes show schizocoelous development, in which the mesoderm proliferates from a single cell and divides to form a mass on each side of the body; the coelom arises from a split within each mass. Deuterostomes show enterocoelic pouching, in which the endoderm evaginates and pinches off discrete pouches, the cavities of which become the coelom and the wall the mesoderm. The animals in these major divisions of the Bilateria differ in other fundamental ways, which are detailed below.

Unlike sessile sponges or floating jellyfish, the Bilateria typically move actively in pursuit of food, although many members have further evolved into sessile or radial forms. Directed movement is most efficient if sensory organs are located at the head or forward-moving end of the animal. Organs of locomotion are most efficiently arranged along both sides, a fact that defines the bilateral symmetry; many internal organs are not in fact paired, whereas muscle layers, limbs, and sensory organs almost invariably are. The diffuse nerve net of coelenterates coalesces into definite tracts or bundles, which run posteriorly from the anterior brain to innervate the structures of locomotion.

Acoelomates
Flatworms (phyla Platyhelminthes, Nemertea, and Mesozoa) lack a coelom, although nemerteans have a fluid-filled cavity at their anterior, or head, end, which is used to eject the proboscis rapidly. The lack of a fluid-filled cavity adjacent to the muscles reduces the extent to which the muscles can contract and the force they exert (see below Support and movement). Because most also lack a circulatory system, supplying muscle tissues with fuel and oxygen can be no faster than the rate at which these substances diffuse through solid tissue. Flatworms are thus constrained to be relatively flat and comparatively small; parasitic worms, which do not locomote, can achieve immense lengths (e.g., tapeworms), but they remain very thin. The larger of the free-living flatworms have extensively divided guts, which reach to within a few cells of the muscles, thus compensating for the lack of a circulatory system. Most flatworms have but one opening to the gut. Nemerteans, in addition to a coelom-like housing for their proboscis, have attained a one-way gut and a closed circulatory system. Both increase their ability to move food and oxygen to all parts of the body. Flatworms are considered to be the ancestors of all other Bilateria.

Pseudocoelomates, or aschelminths
The pseudocoelomates include the nematodes, rotifers, gastrotrichs, and introverts. Some members of some other phyla are also, strictly speaking, pseudocoelomate. These four phyla of tiny body size (many species no larger than the bigger protozoans) are placed together in part because they lack mesoderm on the inner side of the body cavity. Consequently, no tissue, muscular or connective, supports the gut within the coelomic fluid. For tiny organisms, this is advantageous for conservation of tissue: there is no reason to evolve or to maintain a tissue that is not functionally important. The inconspicuousness of most of these phyla has led to a slow advancement in understanding their phylogenetic position in the animal kingdom