Vestigial marginal hooklets in the oncomiracidium of the microbothriid monogenean Leptocotyle minor.

An earlier suggestion based on light microscopy that haptor "spicules" in the oncomiracidium of the microbothriid monogenean Leptocotyle minor are vestigial marginal hooklets is supported by their ultrastructural features. Each spicule is a solid rod of homogeneous, uniformly electron-dense material and is enclosed within a cell (or cells) (oncoblast?). We confirmed that there are six of these spicules in L. minor.