The body of Annelids is divided into a number of segments longitudinally. All the segments look alike. They are called metameres and this is called metamerism. In these segments all systems are repeatedly arranged. Usually the metamerism is confined to the trunk region of the organisms. Cephalic and anal regions may not show metameric nature in the caphalic region sense organs are concentrated, where in the anal region new segments are produced in front of anal segment.

1. Metamerism first observed in Annelida in the animal kingdom.

2. The most successful animals of animal kingdom like arthropoda and chordate will also show metameric segmentation.

3. In annelids the metameric segmentation is both external and internal. The body is divided into a number of segments which contain all body organs repeatedly but the alimentary canal is long and straight tube extending through all the segments.

4. In arthropods the segmentation is external.

5. In chordates the segmentation is internal.

Homonomous & Heteronomous metamerism:

In annelida the body is divided into a number of segments. All are usually similar. If all the metameres are similar throughout the body it is called Homonomous metamerism. But in some groups like arthropoda and chordata the anterior segments will show clear cephalisation. They are modified into head bearing specific sense organs. Such metamerism is called "Heteronomous metamerism.

(Even in annelids some anterior segments look different. Hence typical homonomous segmentation is not seen in any animal.)

In embryonic stages the metamerism is complete and uniform. But in adult condition it will change due to cephalisation.

Origin of metameric segmentation: The origin of metamerism is not clearly known. Many theories were proposed to explain the development of metamerism.

1. Fission theory:

1) Metamerism is derived from non-segmented ancestor, which might have under gone transverse fissions repeatedly and gave metamerism.

2) This theory was improved by Perrier.

3) This theory infers annelids might have developed from Platyhelminthes.


1. Because of fission the organism will divide into separate individuals but they will not unite to form a metameric individual.

2. Reproduction by fission is confined to sessile animals but not in free moving organisms.

2. Pseudometamerism theory:

This theory was supported by Hyman & Goodrich.

According to this theory the body parts like coelom, blood vessels, nephridia muscles etc. will be repeatedly formed. In between them septa are formed. Thus metamerism is derived. This can be seen in some larval forms and adults of some annelids.

3. Embryological theory: In the embryonic  stage by some stress in the mesoderm fragmentation is developed which gave metameric segmentation.

4. Locomotory theory:

This theory is a combination of pseudometamerism theory and embryo­logical theory.

It is believed that metamerism is derived as an adaptation to locomotion:

1. In annelids the segmentation is developed as an adoptation for burrowing.

2. In chordates the metamerism is developed as an adoptation for swimming, undulatory movements.

Most probable annelid ancestors were long coelomate organism. In these animals by the development of septa the liquid skeletons and muscles function will be localised and is advantageous for burrowing. Afterwards the nervous system, excretory system etc. are also undergone segmental-organization.

In chordates the metameric segmentation of body wall and musculative allow alternate waves of contraction which will help in swimming. Thus locomo­tion might have caused metameric segmentation in these animals.

Significance of Metamerism :

1. Metameric segmentation helps the animals in their locomotion.

2. The segments will show high structural development which gave scope for evolution.