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(segmentation), in biology, division of the body in many bilaterally symmetrical animals into more or less similar repeating parts, or metameres (segments), arranged serially along the body’s longitudinal axis.
Metamerism in the form of strobilation is characteristic of parasitic tapeworms. The body of the parasite consists of proglottids, segments of identical structure that bud at the head (neck region) of the worm and form a chain, or strobila. Metamerism may be external only (pseudometamerism), or it may involve the internal organs as well (true metamerism). True metamerism can be complete, involving the entire organism, or incomplete, extending only to a few organ systems (for example, dermatomeres, or cutaneous metameres; myomeres, or muscular; scleromeres, or skeletal; and neuromeres, or neural).
A distinction is made between homonomous metamerism, where all of the metameres are structurally similar, perform identical functions, and bear identical extremities, and heteronomous metamerism, where the metameres, while retaining essentially a common structural plan, differentiate in different directions, become externally dissimilar, and bear different extremities or lose some. Complete metamerism is characteristic of annelids and arthropods, in which the metameres coalesce to form a head, thorax, and abdomen.
In chordates, metamerism is manifested in the structure of the skeleton, musculature, nervous system, cutaneous formations, circulatory system, and excretory organs. In most vertebrates, including man, metamerism is clearly expressed in the early stages of embryonic development. In the human adult, metameric features survive in the vertebral skeleton, cerebrospinal reflex centers, and roots of the spinal nerves and in the regular alternation of ribs, intercostal muscles, and nerves.
B. S. MATVEEV
in chemistry, a special case of isomerism relating to the position of a heterocyclic atom in a chain of aliphatic compounds. For example, methylpropyl ether CH3OCH2CH2CH3 and diethyl ether CH3CH2OCH2CH3 are metameric. The term “metamerism” was suggested by J. Berzelius in 1830. Today it is seldom used.
metamerism(1) In colorimetry, the quality of some colors that causes them to appear different under various light sources. For example, two color samples might appear the same in natural light, but not in artificial light.
(2) In biology, repeating segments that appear the same but perform different functions. An earthworm is a common animal example.