抄録
In coiled mollusc shells, the apex typically is located near the centre of a whorl spiral and the aperture at its outer perimeter (exceptions do occur in molluscs with determinate or periodic growth, but they do not invalidate this general principle). This geometry satisfies simple growth and functional requirements. The Recent vermetid Dendropoma sp. defies these requirements with its inside-out shell geometry, in which the aperture is located along the axis of the spiral and earlier whorls coil around it. In addition, this species is unique among the Vermetidae in being fully endolithic in the adult, and is one of very few endolithic molluscs with the shell cemented to the substrate during growth. While Dendropoma is typically semi-endolithic, several species appear to have secondarily returned to epifaunal coiling as a response to crowding of the substrate. In D. sp., this was prevented by the immediate environment, subjected to a high rate of erosion. This is likely the factor that triggered the onset of endolithic coiling into the substrate. The change from a semi-endolithic to a fully endolithic life habit in this form was probably sudden, since the preadaptations of Dendropoma prevent a functioning intermediate stage.
