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Coelom segmentation, myomeres and the question of the vertebrate antecedents


Coelom segmentation, myomeres and the question of the vertebrate antecedents



Z Zool Syst Evolutionsforsch 4(1/2): 13-57



A hypothetical reconstruction of the predecessors of Acrania and Vertebrates is based on the assumption that as in the Protostomians, the coelom originally was the fluid container of the hydrostatic skeleton (Clark 1964). The "Larval-Tail-Theory" is based on a great number of improbable assumptions: the larvae of sessile and nearly immobile organisms are the predecessors of Acrania and Vertebrates which are animals with a highly efficient movement apparatus; the tail was formed at the rear end of the animal; the somites had their origin in the tail and came into relation with the coelom only secondarily; the enterocoelic formation of a segmental coelom in Acrania and Cyclostomes is interpreted as secondary and without phylogenetic importance; Branchlostoma is not accepted as a representative of the lowest level of Vertebrate evolution. The fact that the somites and the notochord reach the fore end of the body is regarded to be secondary. Contrary to the "Larval-Tall-Theory" the "Worm-Theory" is much simpler. According to it: the Acrania and Vertebrates are derived from actively mobile predecessors. The improbable assumption that they emerged from the larvae of sessile forms is not upheld; the coelomic segmentation in the ontogeny of Acrania and Cyclostomes is interpreted as the morphological construction which allowed the predecessors to swim by sinusoidal movements; the somites (myomeres) are derived from coelomic segments and therefore the relation between coelom and somites is maintained to be primary. Branchiostoma can be regarded as a missing link between the invertebrate level and the vertebrate level of organizations as far as body musculature and coelom segmentation are concerned: the swimming by undulations of the fish trunk is derived from the sinusoidal movements of the whole body: the somites in the head were reduced when this region became immovable. Different Head-Theories are discussed. It is concluded that the Vertebrate Head developed in a region of the body that was characterized by the presence of myomeres. In its development the head with its sense organs lost the segmental (myomerie) organization of the predecessors. The tripartite division of the Vertebrate body (according to the Starck's theory is interpreted as the final stage of the early evolution of the Vertebrates.

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