The structure and histochemistry of the cyst wall of the metacercaria ofFasciola hepaticaL.

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Abstract
The cyst wall which encloses the metacercaria ofFasciola hepaticaconsists of four major layers, one of which is further divisible into three sublayers. These layers are numbered I to IV, the first being the external layer. Layers I and II form the outer cyst which may be separated from the inner cyst formed from layers III and IV.Layer I, the thick incomplete external layer, covers the metacercaria dorsally and laterally. It is composed of tanned protein.Layer II is a thin fibrous layer closely adherent to the inner surface of layer I and is composed of mucoprotein and acid mucopolysaccharide.Layer III is made up of three separate sublayers, the relative widths of which vary in different regions of the cyst. In general IIIaconsists of mucoprotein, IIIbof acid mucopolysaccharide and IIIcof neutral mucopolysaccharide.In the dorsal and lateral regions, layer IV appears to be formed of lamellae held in a protein–lipid matrix. The lamellae are composed of protein stabilized by disulphide linkages.In the ventral region of layer IV there is a thickened mucopolysaccharide area, the ventral plug, through which the metacercaria excysts.The relative importance of these resistant layers in providing protection against desiccation, toxic substances and attack by other organisms is discussed.This work was carried out during the tenure of a Commonwealth Post Graduate Scholarship. The author is grateful to Professor J. D. Smyth and Dr J. A. Clegg for advice and encouragement during the course of this work and in preparation of the manuscript.