Metameres

Metameres are the repeating regional units of an animal's body plan along the head-to-tail axis. Metamerism refers to the serial repetition of similar or identical body segments called metamers. Segments may be externally visible, as in many annelids, or internally organized, as in vertebrate somites. Metamerism can be homonomous, with segments retaining a common structure, or heteronomous, with adjacent segments specializing for different functions.

In annelids such as earthworms and polychaetes, the body is divided into multiple metameres, each bearing elements such as bristles (setae), muscles, nephridia, and a repeating nerve cord structure. The segments share a general plan, but functionally specialized as you move along the body.

In arthropods, segmentation is evidenced by repeated body blocks, each with its own musculature, nerves, and often limbs. Over evolutionary time, segments may fuse into tagmata (such as head, thorax, abdomen) that perform combined functions, though underlying segmentation remains.

In jawed vertebrates, metamerism is most evident in the developmental stage: somites form along the neural tube and later differentiate into vertebrae, ribs, vertebral muscles, and dermis. External segmentation is reduced by fusion, yet the axis-level repetition persists in the embryonic pattern.

Metamerism is a major organizing principle in animal evolution and development, reflecting serial homology and modularity that facilitate growth, regeneration, and functional diversification. Fossil groups such as trilobites display clearly segmented bodies; in other lineages the pattern is concealed or modified by fusion.