Evolutionary Significance of Metamerism
Evolutionary
Significance
of Metamerism
No truly satisfactory explanation has yet been given for the origins of metamerism and the coelom, although the subject has stimulated much speculation and debate. All classical explanations of the origin of metamerism and the coelom have had important arguments leveled against them, and more than one may be correct, or none, as suggested by R. B. Clark.* The coelom and metamerism may have evolved independently in more than one group of animals, as, for example, in chordates and probably twice in protostomes. Clark stressed the functional and evolutionary significance of these features to the earliest animals that possessed them. He argued forcefully that the adaptive value of a coelom, at least in protostomes, was as a hydrostatic skeleton in a burrowing animal. Thus contraction of muscles in one part of the animal could act antagonistically on muscles in another part by transmission of the force of contraction through the enclosed constant volume of fluid in the coelom.
Although the original function of the coelom may have been burrowing in the substrate, certain other advantages accrued to its possessors. Some of these were mentioned in the prologue to Molluscs . In addition, coelomic fluid would have acted as a circulatory fluid for nutrients and wastes, making large numbers of flame cells distributed throughout the tissues unnecessary. Gametes could be stored in the spacious coelom for release simultaneously with other individuals in the population, thus enhancing chances of fertilization, and synchronous release of gametes would have selected for greater nervous and endocrine control. Finally, separation of the coelom into a series of compartments by septa (metamerism) would have increased burrowing efficiency and made possible independent and separate movements by separate metameres, as mentioned in the prologue to this section. Independent movements of metameres in different parts of the body would have placed selective value on a more sophisticated nervous system for control of movements, thereby leading to elaboration of the central nervous system.
No truly satisfactory explanation has yet been given for the origins of metamerism and the coelom, although the subject has stimulated much speculation and debate. All classical explanations of the origin of metamerism and the coelom have had important arguments leveled against them, and more than one may be correct, or none, as suggested by R. B. Clark.* The coelom and metamerism may have evolved independently in more than one group of animals, as, for example, in chordates and probably twice in protostomes. Clark stressed the functional and evolutionary significance of these features to the earliest animals that possessed them. He argued forcefully that the adaptive value of a coelom, at least in protostomes, was as a hydrostatic skeleton in a burrowing animal. Thus contraction of muscles in one part of the animal could act antagonistically on muscles in another part by transmission of the force of contraction through the enclosed constant volume of fluid in the coelom.
Although the original function of the coelom may have been burrowing in the substrate, certain other advantages accrued to its possessors. Some of these were mentioned in the prologue to Molluscs . In addition, coelomic fluid would have acted as a circulatory fluid for nutrients and wastes, making large numbers of flame cells distributed throughout the tissues unnecessary. Gametes could be stored in the spacious coelom for release simultaneously with other individuals in the population, thus enhancing chances of fertilization, and synchronous release of gametes would have selected for greater nervous and endocrine control. Finally, separation of the coelom into a series of compartments by septa (metamerism) would have increased burrowing efficiency and made possible independent and separate movements by separate metameres, as mentioned in the prologue to this section. Independent movements of metameres in different parts of the body would have placed selective value on a more sophisticated nervous system for control of movements, thereby leading to elaboration of the central nervous system.
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