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Animal, any member of the kingdom Animalia, which comprises all multicellular organisms that obtain energy by ingesting food and that have cells organized into tissues. Unlike plants, which manufacture nutrients from inorganic substances by means of photosynthesis, or fungi, which feed by absorbing organic matter in which they are usually embedded, animals actively acquire their food and digest it internally. Associated with this mode of nutrition are many of the additional features that readily distinguish most animals from other life forms. Specialized tissue systems permit animals to move about freely in search of food or, for those that are fixed in place during most of their lives (sessile animals), to draw the food toward themselves. The well-developed nervous systems and complex sense organs that have evolved in most animals enable them to monitor the environment and, in association with specialized movements, to respond rapidly and flexibly to changing stimuli.
Almost all animal species, in contrast with plants, have a limited growth pattern and reach a characteristically well-defined shape and size at maturity. Reproduction is predominantly sexual, with the embryo passing through a blastula phase .
The conspicuous differences between plants and animals first led to a formal division of all life into two kingdoms, Plantae and Animalia. When the world of microorganisms was later investigated, some were seen to be clearly plantlike, with walled cells and chlorophyll-containing bodies that conducted photosynthesis, whereas others resembled animals in that they moved about (by means of flagella or pseudopodia) and digested food. The latter type,
In the five-kingdom system used in this encyclopedia , animals are limited to organisms with differentiated tissues, and the protozoan groups are assigned to the kingdom
The multicellular animals (metazoa), as stated, evidently arose from animal-like, unicellular creatures (protozoa). Precise relationships are unclear because of the poor fossil record and the extinction of intermediate forms, but several evolutionary routes are possible. For example, certain animal-like flagellates occur as colonies and could readily have evolved into more elaborate organisms. In addition, the embryonic stages of some animals display a sequence of changes that provides a reasonable evolutionary model: a unicellular stage, followed by an undifferentiated colonylike stage, a hollow ball of cells (blastula), and then a gut (gastrula stage). Other theories suggest different transitional forms, such as a protozoan with many nuclei in one cell.
Since its uncertain beginnings, the animal kingdom has diversified into several major lineages or branches, which in turn have subdivided into phyla, classes, and smaller groups. The old notion that a single series of organisms has progressed from lower to higher forms, in a so-called chain of life, has long since been rejected. The course of evolution is more comparable to a tree or bush, with many branches undergoing diversification or adaptive radiation, and with some progressive evolution occurring throughout the kingdom. Thus, insects, cephalopods, and vertebrates followed different evolutionary routes, but all of them can be described as “higher” animals.
Much of the diversification within the kingdom probably occurred more than 600 million years ago, before the Cambrian era, because a wide variety of fossils, representing most major animal groups (phyla), already appear in the oldest Cambrian rocks. Because the phyla arose in earlier times for which the fossil record is poorer, relationships between them have to be inferred on the basis of conservative features, including embryology, and are often conjectural. Better evidence usually exists for subdivisions within each phylum, partly because most of these originated more recently.
One basic body part is the gut, which must have arisen early in the evolution of the metazoans. The simplest animals, sponges, have internal cavities that function in feeding, but the openings are not comparable to a mouth or anus. Furthermore, they have tissues but no real organs or nerves, and they lack symmetry. The more complex jellyfish and their allies, more active animals that generally feed with tentacles, have a gut with a mouth but no anus. A nervous system is present, but no brain or head. A jellyfish body also has no bilateral symmetry—that is, no left or right side—as have the bodies of more advanced animals, including humans. Instead, the animals exhibit radial symmetry, or symmetry around a central axis.
A major advance occurred with the origin of a coelom in addition to the gut. This added body cavity allowed for the movement of internal organs. In some animals it may also double as a circulatory system and as a skeleton, if its contents are under pressure. Two basic types exist. A true coelom is a cavity within the mesoderm, covered with a layer of tissue called an epithelium. A pseudocoelom, however, lacks such a covering. A number of phyla of mainly wormlike animals have this latter kind of body cavity and hence are called pseudocoelomates. Such animals generally are of modest size and have poor locomotion. They lack a circulatory system and are simple in various other respects. Nevertheless, they are quite successful in some habitats. The remaining organisms have a true coelom and are called eucoelomates.
The true Schizocoela are related to the annelid worms (phylum Annelida). Metamerism, or segmentation (a serial repetition of groups of body parts), arose in this group, an arrangement that is thought to have allowed better locomotion. The arthropods (phylum Arthropoda) retain segmentation but add a hard external skeleton (exoskeleton) that improves locomotion and provides support and protection. The mollusks (phylum Mollusca) have reduced coeloms and little indication of segmentation, but they have a characteristic shell and soft body.
What follows is a brief summary of the phyla, which are grouped into some of the larger assemblages discussed above. Further information is given in separate articles under the common name of the phyla or superphyla. The number of known animal species, about 1 million, is approximate and does not include fossil species.
The phylum is also called Cnidaria, especially when extended to include the Ctenophora (see below). The stages of a coelenterate's life history include an attached polyp (as in sea anemones), a free-swimming medusa (as in jellyfish), or both. Prey is taken with stinging capsules, generally located on the tentacles. The animals are marine, except for a few freshwater forms such as Hydra. 3 classes and 5300 species.
Two subphyla of invertebrate chordates exist. The Tunicata (tunicates, or sea squirts) become highly modified as adults and attach to a substrate, feeding with gill slits. Only the free-swimming larvae retain a notochord. All are marine, and about 1300 species exist. The Cephalochordata (lancelets) resemble very simple fish. They live in the sea and extract food from the water with their gill slits. About 25 species exist.
The remaining Chordata belong to the subclass Vertebrata and have a backbone. Half of the 42,000 species are fish. Thus, although the group is very successful, it is outnumbered by arthropods and mollusks. Seven classes are generally recognized. The old class Pisces (fish) has long since been broken down into three classes: Agnatha (jawless fish such as lampreys), Chondrichthyes (cartilaginous fish such as sharks and rays), and Osteichthyes (bony fish). The class Amphibia contains such semiaquatic forms as salamanders, toads, and frogs. The class Reptilia is better adapted to life on land; it includes turtles, lizards, and crocodilians. The class Aves (birds) is noteworthy not only for flight but also for warm blood and an insulating layer of feathers. The class Mammalia is characterized by the possession of hair and of mammary glands that secrete milk. This group also is warm-blooded.
Animals have diversified so as to occupy many ecological niches. Herbivores form a basic link in the food chain, or food web. They are eaten by carnivores, which in turn are eaten by other carnivores. Many animals are parasites or scavengers or feed on sediments. Every animal has adaptations, such as sense organs and teeth, that enable it to find and utilize food. Some use a wide variety of foods; others specialize. Features such as camouflage and protective shells enable them to cope with predators.
Animals compete with one another for scarce resources, including food and a place to live. The interactions between predator and prey have involved a coevolution of these different parts of the ecosystem—that is, the evolution of prey is affected by the evolution of predators. Parasites, which are harmful but usually do not kill their hosts, must be able to overcome defensive adaptations. On the other hand, mutualists have cooperative arrangements with other organisms. Plants and animals can also cooperate, in the sense that food is provided by plants, and various animals effect pollination or seed distribution.
A great variety of strategies enable animals to survive and reproduce. When food in an ecosystem is abundant, the animals involved generally tend to reproduce in quantity. When intense competition for food is involved, however, the animals have to develop a more effective use of resources and tend to care for the young for a prolonged period. Where the ecosystem is stable, as in the Tropics, competition leads to complex life cycles and intricate adaptations, both structural and behavioral.
Animals often move from place to place. This enables them to locate food, avoid predators, and reproduce effectively. Through migration, birds and other animals can feed and reproduce where food is seasonally abundant. The movement of animals is limited only by the food supply, the terrain, and their individual capabilities for locomotion, and each local area and habitat has tended to evolve its own distinctive fauna.
Human societies have continued to exhibit traces of these relationships. The aesthetic appeal of animals is expressed in the mosaics and frescoes of ancient Greece, Rome, and Egypt; in the paintings of the Renaissance masters; and in countless works of more recent artists. Mystical feelings toward animals are reflected in myths and folktales, moralistic fables involving animals, and anthropomorphic animal stories for children, as well as the adoption of animal names for cars, sports teams, and so on.
Despite highly developed technology and agriculture and the domestication of many species, modern humans still depend on wild stocks of a variety of other animals, such as fish and shellfish, for food. Unmanaged commercial exploitation, however, has greatly reduced this resource and has driven some species to the edge of extinction . In addition, wild animals provide recreation in the form of sport hunting and sportfishing, bird-watching, and visits to zoos and wildlife refuges.
As humans advanced from a hunting to an agricultural existence, relationships between humans and animals changed. Animals that preyed on livestock or that destroyed crops were reduced or exterminated, and the alteration of the landscape eliminated habitats for many other species. As humans became more concentrated in cities, however, animals such as rats multiplied and became important carriers of disease.
Humans domesticated some animals for food, clothing, power, and companionship. How this came about is a matter of speculation. Through protection and selective breeding, humans changed early domesticated forms into highly productive dairy and beef cattle, sheep, swine, and poultry. Also contributing to human welfare are the dogs, cats, white rats and mice, guinea pigs, and rhesus monkeys that medical research has used to gain an understanding of human physiology and to develop drugs and procedures to combat human illnesses.
As humans continue to spread across the earth, however, they encroach on or pollute the environments of many animals, reducing remaining habitats to smaller and smaller areas. Unless this trend is reversed, much of the world's animal life faces extinction.
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