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The adaptive significance of foot reversal in the Limoida
Gilmour, T.H.J. (1990). The adaptive significance of foot reversal in the Limoida, 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. 249-263
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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  • Gilmour, T.H.J.

Abstract
    The settlement and metamorphosis of pediveligers of one species belonging to the order Limoida are described. It is shown that the rotation of the foot through 180° which is characteristic of the order is probably concerned with balancing the shell and visceral mass on the foot during the crawling movements of the pediveliger. The pedal and byssal retractor muscles of four species in representative genera of the order are shown to be quite different. These differences may be explained by the loss of the original posterior retractor muscles during the early stages of the rotation of the foot and the subsequent development of new retractors when rotation approaches 180°. The changes in the pedal retractors have been accompanied by the loss of the anterior adductor muscle in a unique pattern for the development of monomyarianism in which a transitory bilaterally asymmetrical stage has been followed by a return to symmetry. The rotation of the foot in the Limoida can be compared with the torsion found in gastropods. In both groups a pre-adaptation concerned with stabilizing the shell and visceral mass on the foot during settlement has had profound consequences for many aspects of adult form and function. Rapid growth of the tentacles of the Limoida in order to help to balance the shell and visceral mass has resulted in a unique pallial structure which led to novel defensive adaptations including mucus secretion and autotomy. Relocation of the foot is accompanied by a switch from ciliary to muscular mechanisms for rejection of the pseudofaeces which can be related to the evolution of the swimming and nest-building behaviour and the elaboration of the lips. The enlarged and sometimes partially fused lips collaborate with the peculiar labial palps and gills in an unusual method of food collection and waste rejection which may have led to the exploitation of infaunal and abyssal habitats by some genera of the order.

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