metameren

Metameres, or metamers, are the serially repeated body units that appear along the anterior-posterior axis of many animals. Each metamere is a repeating developmental and anatomical module that can house components of multiple organ systems. The term comes from Greek meta- meaning “after” or “beyond” and meros meaning “part.” Metamerism describes the condition or pattern formed by these repeated units.

Metamerism is prominent in several major animal groups. In Annelida (such as earthworms and leeches) the body is clearly segmented into metameres, each often containing a coelomic chamber, nephridium, nerve ganglion, and paired appendages or setae. In Arthropoda (insects, crustaceans, myriapods) segmentation underlies the organization into repeated body units, though many segments can become specialized into regions known as tagmata, with differing appendages and functions. In vertebrates, a related concept appears in the axial skeleton: somites form sequential metameric blocks that give rise to vertebrae, ribs, and associated musculature, reflecting a vertebrate form of metamerism during development.

Developmentally, metamerism commonly arises from segmentation of mesodermal tissue into paired or sequential units, though the degree of internal specialization varies among groups. Functionally, metamery supports modular growth and variation, enabling segments to diversify or specialize while maintaining an overall body plan. Evolution has repeatedly elaborated or reduced metameric patterns through processes such as tagmatization and regional differentiation.

See also: metamerism, metamere, somite, segmentation.