Metamerism is the condition when the general segmentation of bilateral animals involves longitudinal division of the body into linear series of similar sections. Metamerism is also known as metameric segmentation. Each section is called metamere or somite or segment. Each of these segments has repeats of some or all units of organs. This term metamerism is used only when the organs of mesodermal origin are so arranged. Metamerism is a Greek term meaning meta=after, mere=part.
The primary segmental divisions are body wall musculature and coelom. This in turn imposes a corresponding metamerism on the associated systems. Longitudinal structures like gut, main blood vessels and nerves extend through entire length of the body. While structures like gonads are repeated in all or only few segments.
Metamerism in Animal Kingdom include,
External and internal metamerism: In most of the Annelids, metamerism is conspicuously visible both externally and internally. Example, Pheretima posthuma, it has numerous body segments and all body being repeated segmentally. Moreover even the coelom is segmentally divided into compartments by intersegmental transverse mesenteries called septa. Only the digestive tract escapes this metamerism and it extends through every segment.
In Arthropods, metamerism is chiefly external. Humans and other vertebrates show internal metamerism of nerves, blood vessels etc.
Complete and incomplete metamerism: Complete type of metamerism practically affects all the body systems. In this type the metameres are homonomous and each metamere has segmental blood vessels, nerves, coelomoducts and nephridia. Thus this condition is also called homonomous metamerism.
Metamerism in Arthropods and other higher animals is incomplete because of division of Labour. Consequently metameres of different regions of body vary considerably. Such a condition is called heteronomous metamerism. The larval and embryonic stages of arthropods and other vertebrates show complete metamerism with uniform metameres. But these metameres become unclear in the adults succeeding specialization.
True and pseudometamerism: True segments in Annelids are developed during the embryonic stages whereas the pseudo segments present in tape worms are superficial which are formed as a result of strobilization. The proglottids of tapeworms are not true segments but rather they are complete reproductive individuals.
| True Metamerism | Pseudo Metamerism |
|---|---|
| Number of segments is constant for each species. No new segments are added except in asexual reproduction. | Number of segments is not constant as new segments are constantly added throughout the life. |
| Simple elongation of the preexisting segments results in growth. | Addition of new segments from proliferation region results in growth. |
| All the segments are of same age and at same stage of development. | Proglottids vary from one another in age and degree of development. |
| All the segments are integrated and interdependent functionally. They work in coordination and preserve the individuality of the body. This individuality of the body helps in locomotion. | The proglottids are independent and self-contained units as each of them have full set of sex organs, excretory and nervous systems. Each proglottid is productive unit developed for detachment. |
Pseudometamerism theory: This theory was proposed by Hymen and Goodrich in 1951. This theory explains Pseudometamerism that occurs in Cestodes such as tapeworms. According to this theory, metamerism initially developed secondarily as a result of repetition of body parts like blood vessels, coelom, nerves etc. Later a segmented condition arouse by the formation of cross-partitions between them, so that each segment receives a part of each system.
This process of formation of cross partitions after basic segmentation is also seen in modern day Annelids during development of somites in larval and adult stages. According to this theory it is believed that pseudo segmentation is an adaptation for an undulatory movement.
Fission theory: This theory was proposed by Perrier in 1882. It postulates that pseudometamerism evolved in flat worms by strobilation of body. Strobilation is mainly aimed to increase the rate of reproduction. Proglottids of helminths are serially arranged segments but in reverse order and they increase reproductive capacity many times.
This theory proposes that metameric segmentation resulted when some non-segmented ancestors divided repeatedly by transverse fission or asexual budding to produce chain of sub individuals. Such process occurs even in modern day Annelids and Platyhelminthes animals. Later these sub-individuals integrated morphologically and physiologically into one complex individual. Thus according to this theory a segmented animal is a chain of completely coordinated sub-individuals.
Cyclomerism theory: This theory is proposed by Sedgwick in 1884. According to this theory, metamerism in chordates evolved for better arrangement of organs in coelom. This theory assumes that coelom originated in some ancestral radiate actinozoan coelenterates, through the separation of four gastric pouches from the central digestive cavity. Initial division of two pouches resulted in three pairs of coelomic cavities namely protocoel, mesoscoel and metacoele in ancestral coelomates. Later loss of protocoel and mesoscoel led to unsegmented coelomates like Molluscs. Then the sub division of metacoele produced primary segments, leading to the development of segmented Annelids. This provided septa and compartments in coelom in which organs could be arranged in a better way.
The phylogenic assumption of this theory is that, all bilateral metazoans were originally segmented and coelomate and those acoelomate unsegmented groups like flatworms lost these characters later.
Locomotory theory: This theory is proposed by Clark in 1964. This theory postulates that metamerism evolved as an adaptation to locomotion of different kinds. It evolved independently in chordates for locomotion which was previously carried out by lateral undulation of body in primitive aquatic vertebrates.
Annelid metamerism probably evolved for burrowing. Metamerism allowed myotomes or muscle bundles and nerves to be arranged segmentally for better co-ordination of undulatory movement of body.
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