Metamerism in Annelida

Metamerism in Annelida refers to the remarkable characteristic of these segmented worms, such as earthworms and leeches, where their bodies are divided into a series of repetitive segments. Each segment contains its own set of organs, muscles, and nerves, giving them a unique ability to exhibit incredible flexibility and specialization. This structural adaptation allows annelids to perform complex movements, such as crawling, burrowing, and swimming, with remarkable precision and control. Additionally, metamerism enables the regeneration of lost segments, enhancing their regenerative capabilities. This evolutionary trait has played a crucial role in the success of annelids, as it allows for efficient locomotion, enhanced sensory perception, and functional specialization within each segment, thereby ensuring their survival in diverse habitats and ecological niches.

Metamerism in Different Animals

The body of Annelids is divided into several 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 cephalic region sense organs are concentrated, whereas in the anal region, new segments are produced in front of the anal segment.

1. Metamerism was first observed in Annelida in the animal kingdom.
2. The most successful animals of the animal kingdom like arthropods and chordates will also show metameric segmentation.
3. In annelids the metameric segmentation is both external and internal. The body is divided into several segments which contain all body organs repeatedly but the alimentary canal is a 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 several 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 conditions, it will change due to cephalization.

Origin of metameric segmentation

The origin of metamerism is not known. Many theories were proposed to explain the development of metamerism.

1. Fission theory

1. Metamerism is derived from a non-segmented ancestor, which might have undergone transverse fissions repeatedly and given metamerism.
2. This theory was improved by Perrier.
3. This theory infers annelids might have developed from Platyhelminthes.

Objections

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 to 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, some stress in the mesoderm fragmentation is developed which gave metameric segmentation.

4. Locomotory theory

This theory is a combination of pseudo-metamerism theory and embryological theory. It is believed that metamerism is derived from an adaptation to locomotion:

1. In annelids the segmentation is developed as an adoption for burrowing.
2. In chordates metamerism is developed as an adoption for swimming and undulatory movements.

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

In chordates, the metameric segmentation of the body wall and musculature 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.