metameric segmentation The repetition of organs and tissues at intervals along the body of an animal, thus dividing the body into a linear series of similar parts or segments (metameres). It is most strikingly seen in Annelida. Essentially, metameric segmentation is an internal, mesodermal phenomenon, the body musculature and coelom being the primary segmental divisions; this internal segmentation imposes a corresponding segmentation on the nerves, blood vessels, and excretory organs. In some metameric animals the segmentation is visible externally but in others (e.g. Chordata) external segmentation has been lost and internal segmentation is best seen in the embryo. Metameric segmentation is thought to have arisen as an adaptation to more efficient locomotion.

metameric segmentation (metamerism; segmentation) The division of an animal's body (except at the head region – see cephalization) into a number of compartments (segments or metameres) each containing the same organs. Metameric segmentation is most strongly marked in annelid worms (e.g. earthworms), in which the muscles, blood vessels, nerves, etc. are repeated in each segment. In these animals the segmentation is obvious both externally and internally. It also occurs internally in arthropods and in the embryonic development of all vertebrates, in which it is confined to parts of the muscular, skeletal, and nervous systems and does not show externally.

metameric segmentation The repetition of organs and tissues at intervals along the body of an animal.