What is metamerism in biology?

Metamerism is a phenomenon that is seen in some phyla like Annelida. It is a biological occurrence in which growth occurs by forming linear and fundamentally similar structures. Animals that possess such metamerism are called metamerically segmented. In animals, these metamers are called somites, whereas in plants it called phytomers.

These metamers or somites are formed based on identical structures or fundamental organization.

The first evidence of metamerism: It was first observed in annelids. Each segment usually contains appendages, muscles, nerves, blood vessels, excretory organs, and a pair of coelomic sacs.

The characteristic feature of metamerism:

(i). Metamerism is usually confined to the middle (intermediate) segments and is absent in the head and posterior segments.

(ii). Each metamer is identical as it is the mirror image of the other.

(iii). These metamers are independent of each other.

Types of metamers:

Homonomous and Heteronomous metamerism: If segments are similar throughout the body, it is called homonomous metamerism and if they are not similar, called heteronomous metamerism.

Complete and incomplete metamerism: Metamerism in Arthropods and other higher animals is incomplete because of the division of labor, whereas in annelids, it is complete metamerism as it has uniform metamerism.

External and internal metamerism: Organism in which metamers are only externally visible is called external metamerism, and when it can be visible internally only called internal metamerism.

Significance of metamerism:

(i). Help in effective locomotion

(ii). Provide hydro-static skeletons for burrowing

(iii). Lead to the development of the high-grade organization