Metamerism (biology)

In biology, metamerism is the phenomenon of having a linear series of body segments fundamentally similar in structure, though not all such structures are entirely alike in any single life form because some of them perform special functions. In animals, metameric segments are referred to as somites or metameres. In plants, they are referred to as metamers or, more concretely, phytomers.

In animals, metamery is defined as a mesodermal event resulting in serial repetition of unit subdivisions of ectoderm and mesoderm products. 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.

Metamerism can be divided into two main categories:

In addition, an animal may be classified as "pseudometameric" meaning that it has clear internal metamerism but no corresponding external metamerism as is seen, for example, in Monoplacophora.

Humans and other chordates are conspicuous examples of organisms that have metameres intimately grouped into tagmata. In the Chordata the metameres of each tagma are fused to such an extent that few repetitive features are directly visible. Intensive investigation is necessary to discern the metamerism in the tagmata of such organisms. Examples of detectable evidence of vestigially metameric structures include branchial arches and cranial nerves.

A metamer is one of several segments that share in the construction of a shoot, or into which a shoot may be conceptually (at least) resolved. In the metameristic model, a plant consists of a series of 'phytons' or phytomers, each consisting of an internode and its upper node with the attached leaf. As Asa Gray (1850) wrote:

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