... number of plates is unrelated to the number of gills or kidneys or other multiple organs, indicating that they could not have actually evolved from annelids as they do not show real segmentation. This is quite like the case in pseudometameric animals, which have multiple pairs of organs in a linear arrangement but are not considered to have once been segmented. As for the coelomic body cavity, molluscs do have a coelomic cavity around the heart, but this is more widely regarded as a space that the animals have evolved in which the heart can beat, than the remnants of a more extensive cavity serving as a hydrostatic skeleton. It is mostly accepted that molluscs evolved form flatworms, and probably branched off at around the same time as annelids, before segmentation occurred. They have retained a number of features linking them to flatworm ancestry - they are sometimes described ...

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