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 external segmentation has been lost and internal segmentation is best seen in the embryo.
Endoderm is not involved in metamery. Segmentation is not the same concept as metamerism. Segmentation can be confined only to ectodermally derived tissue, e.g., in the Cestoda tapeworms. Metamerism is far more important biologically since it results in metameres, also called somites that play a critical role in advanced locomotion.