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.
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.
III 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.
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.
Symmetry thus provides a basis for further dividing most animals into two grades: Radiata and Bilateria. The tissues of the Radiata have only two major layers, whereas the Bilateria have three. This came about through the addition of a middle layer (mesoderm) between the outer layer (ectoderm) and inner layer (endoderm). (In the embryological development of animals in general, the ectoderm gives rise to the skin and nervous system, the endoderm to the lining of the gut and some of its derivatives, and the mesoderm to the remaining structures, such as muscles.) Bilateral symmetry is also an evolutionary development linked with an increasing capability of active locomotion, although such locomotion has often been lost in later lines of evolution. Movement in one direction is facilitated by the development of a head, with brain and sense organs at the leading end.
C Protostomia and Deuterostomia
Within the Bilateria, a number of phyla that differ as adults have been linked on the basis of early embryology. Such groups are founded on the basis of how the cells divide and how various organs are formed. For example, one such basic division is into the Protostomia, in which the embryonic mouth persists, and Deuterostomia, in which a new mouth is formed. (Until the Deuterostomia are reached, below, the animals discussed henceforth are Protostomia.)
The ancestral bilaterian had a gut but no other body cavity; because of the absence of such a cavity, or coelom, the animal could be called an acoelomate. It also lacked an anus and a circulatory system but probably had a simple excretory system. Among extant animals, this stage is represented by the flatworms (phylum Platyhelminthes). Lack of a circulatory system and anus makes for an inefficient distribution of materials in the body. The nemertean worms (phylum Rhynchocoela) have a circulatory system and anus, but their locomotion remains slow and clumsy.
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 worm-like animals have this latter kind of body cavity and hence are called pseudocoelomates. Such animals are generally 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.