Abstract: Hydra pseudoligactis was treated with different concentrations of Pb²⁺ and Cd²⁺, and the variations of its surface ultra-structure were observed by scanning electron-microscopy. The results showed that the variation of H. pseudoligactis surface ultra-structure was correlated with the kinds of test ions, their concentrations and exposure time, and generation number of H. pseudoligactis. Under the ion concentrations of ≥0.04 mg Cd²⁺·L^-1 and ≥0.25 mg Pb²⁺·L^-1, the surface ultra-structural damage increased with exposure time, and was irreversible. The epidermis corroded, collapsed and disintegrated, and the epitheliomuscular cells, interstitial cells and nematocysts mostly deformed and decreased in number. Especially for nematocysts, they were continuously cracked with exposure time. The morphologic damage of the same generation H. pseudoligactis surface ultra-structure was more serious under Cd²⁺ than under Pb²⁺ exposure.

Key words: Gnotobiotic culture, Bacterivorous nematodes, Bacteria, Population