White patches of skin that appear early in the development of the shell-less mollusc, Aplysia californica, are composed of aggregations of vase-shaped vesicles, each a single, large cell with an enlarged nucleus. Two layers of collagen, at 90° to each other, surround the vesicle membrane, together with a non-contiguous external layer of muscle. Energy dispersive spectroscopy and electron diffraction of the contents with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) failed to find a signal for halogens (involved with unpalatable skin and, therefore, defence against predation), but did record prominent signals for calcium. Immature vesicles are filled with an homogenous material that appeared to mature progressively, likely by nucleation, into numerous, small spherules, visible by both SEM and TEM. Ultra-thin uranyl acetate stained sections of mature vesicles showed that the spherules had an intracrystalline matrix of radially arranged electron-dense material. The vesicle cell had none of the modifications characteristic of calcium or rhogocyte cell-types, which serve as calcium reservoirs for the movement of this element to sites of shell formation and regeneration. These vesicles are likely involved with the excretion of excess calcium and not, as originally hypothesized, with either defence against predators or calcium flux. In addition, the spherules that fill the vesicles are possibly composed of the less common polymorphs of calcium carbonate, vaterite and amorphous calcium carbonate. Excretion of the vesicle contents appeared to be passive, since vesicles lacked an abundant contiguous layer of muscle or a muscular release valve in the vesicle neck, but did have organized layers of collagen just outside the vesicle membrane. We propose that release of vesicle contents probably occurs when the neck of the vesicle is broken.
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