Segmentation in bilateral animals (like Annelida) which involves a longitudinal division of body into a linear series of similar sections or parts.
Each segment is called a somite or metamere.
Metamerism is only when organs of mesodermal origin are so arranged.
Metamerism is limited to the trunk. Head (Prostomium, bearing brain and sense organs) and Pygidium represented by terminal part of body are not metameres.
New segments originate in front of the Pygidium. Oldest segments adjoin the head.
Developed for the first time in Annelida.
Metameric segmentation evolved 3 times independently – Annelida-Arthropoda, Chordates, Cestodes
Metamerism in Annelida
All body organs — musculature, setae, blood vessels, nerves, ganglia, excretory organs and gonads are repeated segmentally.
Coelom is divided into segmental compartments by septa (intersegmental transverse mesenteries).
Complete Metamerism (Homonomous Metamerism) in Annelida. Other higher animals show incomplete metamerism.
Unlike Arthropods and Vertebrates, metamerism persists in adult stage as well.
| True Metamerism | Pseudometamerism |
| Number of segments is generally constant for each species. New segments are not added to the body after maturation, except in asexual reproduction. Growth occurs by elongation of existing segments. Segments and ends of body have a fixed relationship throughout life. All segments are of same age and at similar stage of development. Segments are functionally interdependent and integrated. Working in coordination, they preserve the individuality of body. e.g. In worms, during locomotion, muscles of each segment contract in a sequence so that rhythmical waves pass over the whole body which facilitates locomotion. | Number of segments (proglottids) forming the body is not fixed as new segments are continually added throughout life. Growth occurs due to addition of new segments from a region of proliferation, just behind the scolex. Proglottids differ from one another in age and in degree of development. Proglottids are independent and self-contained units, each having a full set of sex organs and a portion of excretory and nervous systems. |