Effects of Zn, Cu, Hg and Cd on histological structures of the digestive gland of Biomlphalaria alexandrina and Bulinus truncatus have been examined. The digestive gland of both snails showed remarkable histopathological changes due to the exposure of the two snails to the above heavy metals These changes can be summarized as follows:
1- Low concentration of Cu (0.1 ppm) led to tubular collapse and multifocal degeneration including vaculation and nuclear pyknosis and appearance of vesicles with yellowish spherules in the digestive cells of B. alexandrina; necrosis of some
digestive gland cells and precipitation of copper in the connective tissue surrounding the digestive gland tubules of B. truncatus. At higher concentration of Cu (1.5 ppm) the number of vesicles containing spherules, as well as necrosis and degeneration of the digestive gland cells increased for both snails. 2- Low concentration of Zn (2 ppm) caused destruction of the integumental cells, vaculation and nuclear pyknosis of the digestive cells of B. alexandrina while in case of B. truncatus the space between the integument and tubules was expanded and precipitation of Zn, and vacuoles containing yellowish spherules increased. At higher concentrations of Zn, similar pathological changes appeared as in the case of
Cu for bothsnails.  3- The digestive gland of both snails when exposed to low concentrations of Cd and Hg, showed rermarkable histopathological changes involving, tubular deterioration, multifocal degeneration with vaculation and nuclear pyknosi.