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 towards 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 (see Embryology).
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 micro-organisms was later investigated, some were seen to be clearly plant-like, 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 plant-like but had close relatives that were animal-like (Flagellates). 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 classification system used in this encyclopedia animals are limited to organisms with differentiated tissues, and the protozoan groups are assigned to the kingdom Protoctista. 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.
II 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 colony-like 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.