Segmentation - Metazoa

Segmentation or metamerism is the division of the body into smaller transverse compartments along the anterior-posterior axis. Segmentation is widespread among animals, with true segmentation occurring in annelids, arthropods and most chordates though some other groups show superficial segmentation of ectodermal body wall. Fundamentally there are three body forms. First, monomeric where there is no division of the large body cavity at all. Ascaris has this type of body form. Second, oligomeric where the body cavity is divided into three and each region has a separate body cavity hooks with no divisions on the abdomen Phorona is a warm with this body plan'. Third metameric in which the body is divided into head, thorax and abdomen and where abdomen is further divided into a chain of segments.

                                                        Figure: superficial segmentation seem in tape-worms

A Segmented body forms can be seen in tape worms, annelids arthropods and' chordates. Of these segmentation in tape worms is quite different from that seen in the others. We can observe that segmentation in tape worm is superficial, a series of ring like creases develop in the cuticle and the body wall which facilitate bending and telescoping of the body. But this segmentation is strictly ectodermal and this segmentation is a reproductive adaptation. The segments of the entire body are in a continuous process of being produced matured and discarded. The new segments grow in the neck region and the older ones are detached from the posterior end, Each segment functions as an independent unit without having' any vital connections with the other.