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Stomach structure, classification and evolution of the Bivalvia
Purchon, R.D. (1990). Stomach structure, classification and evolution of the Bivalvia, in: Morton, B. (Ed.) The Bivalvia: Proceedings of a Memorial Symposium in honour of Sir Charles Maurice Yonge (1899-1986) at the 9th International Malacological Congress, 1986, Edinburgh, Scotland, UK. pp. 73-82
In: Morton, B. (Ed.) (1990). The Bivalvia: Proceedings of a Memorial Symposium in honour of Sir Charles Maurice Yonge (1899-1986) at the 9th International Malacological Congress, 1986, Edinburgh, Scotland, UK. Hong Kong University Press: Hong Kong. ISBN 962-209-273-X. 355 pp., more

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    Marine

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  • Purchon, R.D.

Abstract
    The classification of the Bivalvia demands an understanding of the functional issues involved in the creation of a new, filter-feeding model, from the original deposit-feeding ancestors, and the subsequent adaptive radiation of this new model to habitats previously unavailable, This first concerned the adaptation of the ctenidia to a new function, and then changes in connection with the ensuing adaptive radiation. Evolution and adaptive radiation necessitate changes that were sometimes not in line with each other. A review of bivalve organ systems has revealed that, with regard to classification, only a few are relevant throughout the class, and recent successes in classifying the Bivalvia have been obtained by using organ systems in pairs to tabulate the superfamilies according to the similarities shown. By doing this several times, with different pairs of organ systems, it was found that many superfamilies always appeared in the same conjunction, in about eight groups, establishing that evolution had occurred concurrently in all of these systems. Two of these systems showed alternative character-states that can be arranged as a progressive change from more primitive to more specialized conditions. These are the ctenidia (filibranch, pseudolamellibranch, eulamellibranch, septibranch) and the degree of fusion between left and right mantle margins. When these two organ systems were employed together they automatically threw out the most primitive (the Pteriomorphia) and the most specialized (the Anomalodesmata and Septibranchia). In this way a number of suprafamilial groups were recognized, e.g., the Pteriomorphia, Unionoida, Veneroida, Myoida and Anomalodesmata, but with exact boundaries between them as yet undefined. The constitution of each group was next analysed more exactly by tabulating their common features under each superfamily, and adding up, this process established which superfamilies shared the largest number of common features and could therefore be assigned to the group in question. The final step, yet to be taken, is to consider evidence which may indicate the superfamilies which could be added to, or taken from, the group, on grounds of convergent evolution. We would then have a system of classification which is based on high similarities not phyletic affinity, and which states what evidence has been taken into account.

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