Segmentation

Segmentation in biology is the division of some animal and plant body plans into a series of repetitive segments. This article focuses on the segmentation of animal body plans, specifically using the examples of the taxa Arthropoda, Chordata, and Annelida. These three groups form segments by using a "growth zone" to direct and define the segments. While all three have a generally segmented body plan and use a growth zone, they use different mechanisms for generating this patterning. Even within these groups, different organisms have different mechanisms for segmenting the body.

Definition and process

Segmentation is a difficult process to satisfactorily define. Many definitions exist, with most agreeing that segmentation involves replication of internal and external features, but disagreeing on other aspects. In 1977, Richard Dawkins stated that "Segmentation is not so much a phenomenon as a problem. There is no one phenomenon called 'segmentation'."

Segmentation in different taxa

Arthropoda

Arthropods are the most visibly segmented animals, with each segment possessing a pair of jointed appendages. Arthropods are covered with a cuticle made of chitin and proteins, secreted by an underlying epidermis. The cuticle provides both protection and rigidity.

Chordata

In chordates, segmentation is subtle. The most easily observable segments are the somites in the developing vertebrate embryo. In the adult, the segmentation of the body plan is the vertebral column.

Annelida

Annelids are segmented worms and their body is externally and internally divided into well-defined segments. The segmentation is both by the body wall and the coelom (body cavity).

Evolution of segmentation

The evolution of segmentation is poorly understood because we do not know whether segmentation is a derived trait or whether it is ancestral. Some researchers see segmentation as a basic body plan, whereas others see it as a complex derived trait.

See also

• Metamerism (biology)
• Modularity (biology)
• Phylogenetics
• Evolutionary developmental biology

References

Segmentation Resources
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