An account has been given of the egg variability, egg development, egg hatching and behaviour and anatomy of the oncomiracidia of three species of the genus Quadriacanthus, monogeneans parasitizing the gills of the catfish Clarias gariepinus inhabiting Nile Delta waters in Egypt. Each of the three species of the genus Quadriacanthus produces eggs of different shapes and sizes. Quadriacanthus clariadis clariadis and Q. allobychowskiella produce four types of eggs while Q. aegypticus produces nine types of eggs. All types of eggs differ from each other in the presence or absence, size and shape of the appendages. It is suggested that eggs of different types are not genetically determined or functionally different. There is no significant increase in the egg size during incubation. Fluid-filled sacs are found only in the fully developed eggs of Q. c. clariadis and Q. aegypticus; they are absent from eggs of Q. allobychowskiella. It is suggested that opening of the operculum and emergence of the oncomiracidium are brought about by a mechanical, chemical and/or osmotic hatching mechanism. The glandular system of the oncomiracidia of the genus Quadriacanthus includes two anterior median head glands, at least six lateral head glands constituting the anterior adhesive apparatus and four posterior body glands. Two crescent-shaped sclerites, representing the precursors of the ventral hamuli, are present in the haptor of the oncomiracidium of Q. allobychowskiella but these sclerites were not observed in the oncomiracidia of Q. c. clariadis and Q. aegypticus.
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