Introduction & Types of Metamerism

Introduction :-

In some animals, the body is divided into a series of compartments arrange one behind to the other. These compartments are called segments or somites or metameres. The Linear arrangement of segments one behind the other is called Metamerically segmented. 

Example :- Annelida, Arthropoda & Chordata . 

Types of Metamerism :- 

(i) True Metamerism :- 

True Metamerism is found in Annelids, Arthropods & Chordates. In true metamerism, the segmentation of the body is based on the segmentation of mesoderm. New segments are formed at the posterior end (in front of the anal segment ). Hence the youngest segments occur at the anterior end. The segments work in co- operation with all other segments.

(ii) Pseudo Metamerism / Superficial Metamerism :-  

Pseudo metamerism is exhibited by tape worms(Platyhalminthes). Here the segmentation of the body is based on the segmentation of ectoderm. New segments are formed at the anterior end (just behind the scolex). Hence the youngest segments occur at the anterior end & the old segments occur at the posterior end. The segments works as independent units. These is no co-operation between the segments.

(iii) Homonomous Segmentation :- 

If all the segments of an animals are identical, the segmentation is called homonomous segmentation. The homonomous segmentation is not found in any existing animal as at least a few of the anterior segments are always specialization is called Cephalization. The closest approach to homonomous segmentation is found in some polychaetes in which all the segments between the head and the anal segment are alike-Homonomous segmentation is a primitive condition.

(iv) Heteronomous Segmentation :-   

If the segments are dissimilar, the segmentation is called Heteronomous segmentation. 

Examples :- Arthropods & Chordates.

(v) External Metamerism :- 

In Arthropods the metamerism is external. Internally the segments are not marked by partitions.

(vi) External & Internal Metamerism :- 

In Annelids the segmentation is marked externally as well as internally. Internally, the segments are separated by transverse partition called Septa.

(Vii) Complete Metamerism :- 

When segmentation is seen in all the organs, the metamerism is called Complete Metamerism.

Example :- Annelida

(viii) Incomplete Metamerism :- 

When segmentation is not seen in all organs, the metamerism is called Incomplete Metamerism.

Examples :- Arthropoda & Chordata