In clean water, the molluscs Anodonta, Unio, and Sphaznuim periodi- 

 cally open the valves and protrude both foot and syphons. In toxic media, 

 the valves are either permanently closed or widely opened and the syphons 

 are protruded. In the gastropods, the reactions to toxicants are quite 

 diverse and peculiar. V-ivipaAuA vlv-ipasiu* in toxic median retracts the 

 body deeply into its shell, tightly shuts the lid and becomes enveloped in 

 a thick layer of musilage. The large pond snail, Llmnaza &tagnata>, ac- 

 tively grazes on plants and filters suspended particles in clean water, 

 its body periodically protruding from its shell. The defensive reflex 

 consisting of retracting the body into the shell as a result of external 

 stimuli is clearly expressed in this mollusc. It normally produces a 

 tape-like excrement, however, the water in the vessel remains clean and 

 transparent, evidently owing to bactericidal effect of the excreted musi- 

 lage and possibly to antibiotic substances. In toxic media, this mollusc 

 does not feed, or the intensity of ingestion of food is greatly dimini- 

 shed, and the body is hidden in the shell. Other essential disturbances 

 in behavior, notably in sexual behavior (Bereza, 1973), are evident. 

 Under acute intoxication, this mollusc falls out of its shell. 



The sensitivity of tests on survival of invertebrates may be consider- 

 ably increased when experiments are conducted at elevated temperatures 

 (Table 5). The sensitivity of biological tests is further increased with 

 sharp changes in temperature. 



When undertaking such testing, it is necessary to consider a number 

 of factors, including: 1) hydrochemical peculiarities of water, its 

 oxygen content, pH; 2) the degree of adaptation of the test organisms to 

 the experimental conditions, their lipid content, age, sex, developmental 

 stage; 3) temperature; and 4) the degree of pollution of the habitat. 



However, in cases without significant statistical differences between 

 the experiment and the control, and the test is of short duration, all of 

 these conditions are of secondary importance, especially when drawing con- 



TABLE 5. INCREASE IN SENSITIVITY OF BIOLOGICAL TESTS ON SURVIVAL 

 OF AQUATIC ORGANISMS AT 30 °C 



Toxicant Test Organisms 



Lethal Concentration, mg/1 



ZnS04*7H20 Vaphula magna 



CdS04 (females with eggs^ 



CuS04*5H20 Vaphnla magna 



ZnS04-7H 2 (young) 



ZnS04-7H20 AazIZu* aquaticuA 



CdS04 AitlZiL6 aquaXxcuA 



109 



