Friday, July 18, 2014

Theories of evolution of Metamerism



                            METAMERISM (With reference to phylum annelida)



 
Metamerism is the division of body into Anterior-posterior rows forming similar segments called somites/metamere. Each metamere typically has repetition of body organs. The term metamerism is only applicable when organ of mesodermal origin are arranged so. Organs like nephridia, blood vessels, reproductive bodies etc.

Types of Metamerism
a) Homonomous metamerism: When every segments of the body have similar organ arrangement. This type of metamerism does not exist in any animals.

b)                        Heteronomous   metamerism: When each segments of the body have different organ arrangement.

THEORIES OF METAMERISM EVOLUTION

CORM AND FISSION THEORY-  

According to this theory the existence of segmented animals occurred when the non segmented ancestors underwent asexual transverse fission. A chain of zooids are formed which remain united from end to end due to incomplete separation. This occurs in some platyhelminths and annelids even today. In the passage of time these zooids got integrated both morphologically are physiologically to form a segmented individual. This theory was proposed by Perrier (1882). But had certain drawbacks.                  
A)  No gradation of age was found.
B)   Asexual transverse fission occurred in sessile animals but the ancestors of annelids were probably free swimmers.

Pseudometamerism:

  This theory was proposed by Hyman (1951). This theory suggests that metamerism evolved secondarily due to the repetition of body organs like blood vessels, nephridia, reproductive bodies and nerve cord etc.
Because of this repetition of organs it resulted in segmentation of body by the formation of cross-partition between them, thus each segments will receive a part of the repeating organ. This process is observed in somite formation of larvae and in some adult annelids even today, this type of metamerism was thought to be an adaptation to swimming mode of life. But all ribbon like animals can swim whether they are segmented or not.

C YCLOMERISM:


This theory was originally proposed by Sedgwick (1884) and later it was supported by Remane (1950, 63). According to this theory 4 gastric or enterocoelic pouches arise from the digestive cavity or gut. Out of the 4 pouches two of them got divided to form 3 pairs of coelomic cavities – Protocoel, Mesocoel, and Metacoel.  The loss of protocoel and mesocoel lead to the existence of non segmented coelomic animals such as mollusca.    
Later the metacoel got segmented primarily giving rise to segmented annelids. The phylogenetic implication of this theory is that the bilateral metazoans were originally segmented and coelomate, but the acoelomate and non segmented groups (flat worms) lost these characters primarily.



Embryological theory:

According to this theory the existence of segmented animals was an embryological accident. Due to the elongation and growth the of larvae/embryo the mesoderm of it was under a lot of mechanical stress which made it fragmented. Later when they larvae grew into adult it got segmented.

LOCOMOTION THEORY:

This theory was proposed by R.B Clark (1964). According to this theory the segmentation of annelids was an adaptation to locomotion and burrowing. The ancestors of annelids used to burrow marine sand and mud. Their coelom was filled by fluids that served as hydrostatic skeleton that helped in burrowing but it hampered locomotion. So the body was divided into septa to provide flexibility. One part of the body can contract while other part of the body remains relaxed. Thus metamerism in annelids initiated from the division of coelom and later the whole body got segmented.




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