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Metamerism in Annelida
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Metamerism in AnnelidaMetameric segmentation or metamerism is an architectural body plan in some animals in which
the similar body segments and organ systems are serially repeated one after another. The similar
body segments are called metameres or somites.
The animals which exhibit such features called metamerically segmented. Structurally each
meta-mere or somite is constructed on the basis of some fundamental plan and usually possesses
a part of almost all the body systems.
In all respects the segments forming an individual are identical in structure and the organs they
contain or in other words, the individual is formed by rhythmic repetition of segments which are
similar both externally and internally. This repetition is often disturbed by simplification, by
coalescence of segments or by .differentiation between the segments.
Occurrence of Metamerism:
Metamerism is first seen in annelids in animal kingdom. Each segment usually contains
appendages, muscles, nerves, blood vessels, excretory organs and a pair of coelomic sacs which
are repeated in almost all segments. It is also seen in kinorhynchs, arthropods and most
chordates.
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.
, Characteristic Features of Metamerism:
1. Metamerism is always confined to the intermediate (trunk) segments except the anterior acron
(head) and a poste rior pygidium or telson.
2. Each metamere represents a mirror image of the other.
3. Segmental structures are interdepend ent on each other.
4. They are integrated into a single func tional unit.
5. All the segments of body work in co ordination.
Types of Metamerism:
The metamerism in different groups is divided into the following types:
1. True Metamerism:
The true metamerism is one in which the segmentation of the body develops by the segmentation
of the mesoderm. It occurs in annelids, arthropods and in most chordates. The body of annelids
consists of a number of segments and the number remains constant in a particular species except
in certain cases of asexual reproduction. New segments are not added to the body after
maturation.
That means after the embryonic stage all the segments become the same age. Moreover, the
segmental structures are inter dependent and integrated so that the individuality of the body is
preserved. New segments arise at the posterior end in front of the pygidium. Hence, newer
segments occur at the posterior end and older segments remain just behind the head.
2. Homonomous Metamerism:
If the segments or somites of the animal are all alike, the’ segmentation is called homonomous
metamerism. It is seen in annelids. Each metamere contains segmental blood -vessels, nerves,
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