Metamer

Homonome metamerization of an anelid.

Heteronome metamerization of an Insect.

Seudometry of a Cestodo.

The metamer is known as each of the segments that are repeated in certain groups of animals, coelomates with bilateral symmetry (bilateria). Metamerization is one of the main modifications of the coelom. Each metamer has coelomic cavities separated from those of other metameres by partitions, and the internal (nerve ganglia, nephridia, gonads, etc.) and external (legs, gills, etc.) structures are repeated in each metamer.

For example, in arthropods, each metamere can carry at least one pair of appendages, which can be legs, antennae, or gills in the case of aquatics. An illustrative example is the centipede (Chilopoda): each of the segments bearing a pair of legs is a metamere; while millipedes (Diplopoda) have two pairs per metamer.

Metamerization also occurs in a characteristic way in the edge of annelids and even in vertebrates, although in this last group it is not so easily seen because throughout the evolution of the group there have been multiple modifications, mergers, reductions, etc. of these metamers and the external metamery has been lost (but not the internal one as evidenced by fish). In many phyla of worms there are cephalized metameres called prostomium containing nerve ganglia arranged in a ring around the initial tract of the digestive tract, which can already be considered a brain: a system of integration for stimuli and impulses formed by nerve cells. It has been possible to obtain conditioned responses to a stimulus, thus demonstrating the possibility of learning by oligochaetes such as earthworms or even more complex animals such as leech (hirudínea), capable of hunting and preying on large fish.

Types of metamerization

• Homonome metamerization. All methammers are similar (e.g. in most of the anelids). It is considered the most primitive. In each methammer or somito there is a repetition of mesodérmic organs, such as gonads.
• Metamerization heteronoma. Metamers are different in different areas of the body; several similar and different segments of others form a tagma (e.g. head, chest and abdomen in insects).
• Seudometry. Some animals present pseudometry or false internal or external metametry; their appearance is similar to that of metametry, but does not derive from jealousy, since only certain organs are repeated, as it happens in the cestodes.

In plants

Plants also exhibit metamerization.