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In biology, metamerism is 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 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:
- homonomous metamery is a strict serial succession of metameres, of which, in fact, there are no true examples in the invertebrates; however, the Annelida worms, e.g., nereis, are used as a model to portray homonomous metamery.
- heteronomous metamery is the condition where metameres have grouped together to perform similar tasks. The extreme example of this is the insect head (5 metameres), thorax (3 metameres), and abdomen (11 metameres, not all discernible in all insects). The process that results in the grouping of metameres is called "tagmatization", and each grouping is called a tagma (plural: tagmata). In organisms with highly derived tagmata, such as the insects, much of the metamerism within a tagma may not be trivially distinguishable. It may have to be sought in structures that do not necessarily reflect the grouped metameric function (e.g. the ladder nerve system or somites do not reflect the unitary structure of a thorax).
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.
See also Edit
|Early Embryonic Development|
|Fertilization - Egg activation - Clevage - Gastrulation - Regional specification|
|Late Embryonic Development|
|Endoderm||Neurulation - Neural crest - Eye development - Cutaneous structure development|
|Other||Limb development - Germ line development - Programmed cell death - Stem cells|
|Post Embryonic Development|
|Metamorphosis - Regeneration - Aging|
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