Metamerism

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Earthworms are a classic example of biological homonomous metamery - the property of repeating body segments

Biological metamery is one of the most important evolutionary advances made in zoology and botany.

In animals

Metamery in animals is defined as a mesodermal event resulting in serial repetition of unit subdivisions of ectoderm and mesoderm products^[1]. Endoderm is not involved in metamery. It should be noted that 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.

The two categories of metamerism are homonomous metamery and heteronomous metamery. 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., earthworms, 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, and abdomen. Humans also have the same grouping of metameres. The process that results in the grouping of metameres is tagmitization, and each grouping is called a tagma (plural, tagmata). (Prof. Jerry L. Kaster, Great Lakes WATER Institute, University of Wisconsin-Milwaukee)

In plants

All land plants produce parts as metamers. Unlike the situation in animals, the metamers of plants extend across all organ systems and tissues within those organs. A single metamer will include epidermal, cortical, and vascular tissue.

References