Metameric segmentation or metamerism is an architectural body plan in some animals in which the similar body segments and organ systems are serially repeated one after another. The similar body segments are called metameres or somites.
Example
1.Annelida possesses both external and internal segmentation.
2. In Arthropoda, segmentation is mostly external.
3. In chordata, segmentation is mostly internal in the form of arrangement of muscle cells and bones of vertebral column.
Characteristic Features of Metamerism: 1. Segments are laid during embryonic development. 2. New segments are not added to the body after maturation. 3. Segmental structures are interdependent on each other. 4. They are integrated into a single functional unit. 5. All the segments of body work in coordination.
In human, muscles are metamerically- segmented as all muscle fibers are formed during embryonic development only. Moreover, even during extensive physical training the number of muscle fibers remains the same but they increase in size as a result highly developed muscles are seen as abs or biceps. One more thing, in their primary arrangements, muscle fibers are arranged end to end (Ref: Anatomy of skeletal muscle). Later their more complex arrangements give rise to a complete muscle.
Likewise, vertebrae are laid during embryonic development only and are arranged end to end. Some of them fuse in later stages.
Pseudo-segmentation: In tapeworm (Platyhelminthes), proglottids (segments of tapeworms) are budded off from neck and are not of embryonic origin. Such segmentation is pseudo-segmentation.
