Animals
    
    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, protozoa, was categorized as a subkingdom of Animalia. Difficulties arose, however, with many forms that showed mixed characteristics and with groups in which some organisms were plantlike but had close relatives that were animal-like. Eventually, classification schemes with several kingdoms were proposed, in which the definitions of Plantae and Animalia became more restricted. What constitutes an animal, therefore, depends on the scheme followed.
    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 Protista. The separation of the protozoa from the higher animals is not entirely satisfactory, however, because classification systems should reflect evolutionary relationships, and multicellular forms are believed to have evolved from protozoan ancestors more than once. Moreover, some protozoans form colonies that are difficult to distinguish from simple multicellular animals. The problem in deciding on the limits of the Animalia is a reflection of the natural world, in which boundaries are blurred and evolution leaves intermediates between major groups in its path.
    Origins and Relationships
    
    
    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.
    Body Organization
    The various kinds of animals have body plans that can be interpreted both historically and functionally. Thus, comparative anatomy enables scientists to sort animals into major groups and to trace and explain their evolution.
    Gut
    
    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. ............