In solutions of chlorophos, initial symptomology is manifested by 

 coiling of posterior segments, and twisting them into a spiral. Grad- 

 ually, twisting of the whole body into a tight spiral occurs, so that the 

 movement ceases (Figure 2, 5-7). The leech subsequently straightens, the 

 gullet opens and swallowing of air occurs. Both volume and weight of the 

 organism increases (Figure 2, 8). 



In a solution of phenol, initial disorderly locomotor activity and co- 

 ordinative disturbances are followed by a looped attachment to the walls 

 of the vessel (Figure 2, 9). The leech then drops to the bottom, convul- 

 sions develop and characteristic constrictions appear in the body (Fi- 

 gure 2, 10-12). Such observations enable an estimation of the character 

 and specificity of action of a harmful compound, suggest a course of study 

 of actual functions responsible for the development of the pathological 

 process, and enable classification of the intoxications. 



The most important problem presently facing aquatic toxicology is the 

 question of adaptation of the organisms to a new environmental factor, 

 the toxicants. Because of their active relationship with the environ- 

 ment, aquatic animals can avoid the harmful effects of such factors, 

 though defensive behavior incorporating the avoidance reaction. 



The ability of fish to avoid toxic solutions under laboratory condi- 

 tions has been widely considered (European Inland Fisheries Commission, 

 1965; Hansen ztal., 1972, 1974; Ishio, 1965, 1969; Jones, 1951, 1957, 

 1964; Shelf ord, 1971; Sprague and Orury, 1969). The experiments have 

 been performed using many species of fish (carp, crucian carp, minnow, 

 loach, trout, etc.) and various toxic compounds: cyanids, phenols, salts 

 of zinc and copper, carbonic acid, ammonium, chlorine, hydrogen sulphide, 

 pesticides, detergents, and industrial wastes. In general, the results 

 of the experiments have shown that various species of fish avoid the 

 zones with sublethal concentrations of toxic substances. 



Ishio suggests that the avoidance reaction obeys the law of Weber- 

 Phekhner, i.e., the response is proportional to the logarithm of the irri- 

 tant intensity (concentration of the toxic substance). However, this 

 reaction varies widely, depending on the species of fish, and the chemi- 

 cal properties of the toxicants. Its manifestion may be strong, weak, or 

 entirely absent. For example, phenol is actively avoided by the carp, 

 but the contaminant is not avoided, even in lethal concentrations, by 

 salmonids. Moreover, some toxic substances are even preferred by fish. 

 Thus, low alkalinity, ammonium hydroxide, and low concentrations of 

 copper salts possess attracting properties. The reaction of trout to 

 dissolved chlorine is most interesting (Sprague and Drury, 1969). A dis- 

 tinct avoidance reaction is observed in the range of low (0.001-0.01 mg/1) 

 and high (1 mg/1) concentrations, but in the range of medium concentra- 

 tions, a clear preference is revealed. It is difficult to explain the 

 difference in reactions. Studies of associated physiological mechanisms 

 are still lacking. However, perception and discerning of toxic substances 

 is largely provided by organs of smell and taste, and to the so-called or- 

 gan of general chemical sense. For example, fish can discern phenol and 



127 



