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The Functions-Coelom theory in the evolution of Mollusca


The Functions-Coelom theory in the evolution of Mollusca



Syst Zool 17(2): 192-208



All discussions hitherto on the origin of the coelom have postulated a reduction within the molluscs; moreover, the recent discoveries of Tryblidiacea (Neopilina) have led persistently again to the opinion that the molluscs should have developed from metamerically segmented ancestors. But, as there are neither ontogenetical nor anatomical reasons, the reduction-theory cannot be upheld. The whole organization of the Mollusca (including locomotion, muscular system, mesenchyme, digestive apparatus, nervous system larval features etc. proves on the other hand that this evolutionary stem must have developed from turbellariomorphic ancestors. The continuity in the locomotion of the molluscs brought from these flatworms guarantees that they did not need a hydrostatical coelom for locomotion (as in other coelomates) and that, therefore, such a coelom has never existed. According to the Functional Coelom Theory the coelom (like every organ) is provided with a special function, and ontogeny as well as the morphological structure of the molluscs refers to the statement that the molluscan coelom has developed as a purely protective pericardium; comparable situations within other animal groups confirm this opinion. The enclosure of the germ cells in the rostral part of the coelom has led to a gonopericardium with double function, and further to a separation in the working organ: pericardium (with coelomoducts) and gonocoel (with gonoducts). The coelomic cavity of the molluscs therefore proves its own development, which is homologous with that of coelomates by way of the cleavage-cell 4d and the derived mesodermal bands. The gonopericardial room itself, however, cannot be considered as a homolog of the secondary body cavity, so that there is a functionally effected diphyletic development of the coelom within molluscs and other coelomates.

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