From Big Medical Encyclopedia

BILATERALISM (bilateralism, bilateral symmetry; lat. bi[s] twice + lateralis side) — the type of symmetry caused by existence of the median plane of the symmetry dividing a body into the left and right half, similar each other, and connected with a differentiation of the back and belly party, and also the front and back ends of a body.

B.'s emergence is connected with transition from the attached or floating way of life to crawling on solid substrate. At a new way of movement there are distinctions in a structure and function of the party turned to substrate (belly), and opposite (back). At the same time need of progress defines distinctions in the organization of the front and back ends of a body. On the front end various sense bodys providing orientation in the environment and a mouth concentrate that creates optimal conditions for capture of food. In relation to back and belly surfaces lateral faces in these conditions are defined as left and right in this connection through an axis of symmetry it is possible to carry out only one plane dividing a body into two symmetric half.

The bilateralism is most characteristic of metaphytes. Having arisen in group of worms, B. remains then to some extent at all more high-organized types. At the lowest multicellular (coelenterates) and protozoa (some flagellates of Polymastigina group, an infusorian of a suborder of Hypotrieha) B. is observed seldom. Emergence of bilateral symmetry in multicellular played an important role in the course of evolution. B. laid the foundation for concentration of sense bodys and nervous elements on the front end of a body and created premises to formation of a brain, provided (unlike a radial symmetry) a possibility of movement in space in several ways (crawling, swimming) and lives on the land. Ecological value B. is defined by the equality of pressure upon both half of a body (left and right) interfering transition of progress of a body to circular.

See also Asymmetry , Symmetry .

Bibliography: Beklemishev V. N. Bases of a comparative anatomy of invertebrates, t. 1 — 2, M., 1964.

T. N. Ulissova.