124 BEHAVIOR OF THE LOWER ORGANISMS 



the details, especially from a quantitative standpoint, vary in accordance 

 with the environmental influences acting upon the individuals in question. 



As a rule, infusoria collect in solutions of substances which may 

 serve them as food. This is almost invariably true for substances which 

 form the usual food of the organism under natural conditions. When 

 the amount of oxygen present in the water is low, most infusoria collect 

 about bubbles of air or other sources of oxygen. 



Infusoria sometimes gather in substances which do not serve for 

 food or respiration, but which serve other important purposes in the 

 physiology of the species concerned. Thus, the flagellate spermato- 

 zoids of ferns were found by Pfeffer to gather in solutions of malic acid. 

 This substance is found in the fern prothalli, and probably occurs in 

 the mouth of the archegonium, into which the spermatozoids must enter 

 in order that fertilization may take place. The tendency to collect in 

 malic acid then doubtless plays a part in bringing about fertilization in 

 ferns. The collection of Paramecia in carbon dioxide seems to be an- 

 other case of a reaction which is useful to the organisms, though the 

 substance causing it does not itself serve as food. 



Many infusoria collect, under certain circumstances, in substances 

 which do not serve as food and are not known to play any useful part 

 in the biology of the animal. Thus, Pfeffer found that the flagellate 

 Bodo saltans gathers in most of the salts of potassium, as well as in various 

 salts of lithium, sodium, rubidium, caesium, ammonium, calcium, stron- 

 tium, barium, and magnesium. This signifies only, as we have already 

 seen, that they are less repelled by solutions of these substances than 

 by the fluid in which they are situated. In most cases, as soon as 

 a substance is sufficiently concentrated to be injurious it becomes 

 repellent. 



Whether the repellent effect of chemicals is due to the chemical 

 properties of the solution, or to its osmotic pressure, has been rigidly 

 determined only for Paramecium. In this animal, as we have seen, the 

 osmotic pressure is usually not the cause of the reaction. There is much 

 evidence that this is true for most species, but accurate quantitative 

 evidence is needed on this point. 



4. Reaction to Heat and Cold 



Infusoria in general react to heat and cold in much the same way as 

 does Paramecium, — through the avoiding reaction. The way the reac- 

 tion occurs is most easily seen in the Hypotricha. The phenomena to 

 be observed are of special interest, because they show clearly how a 

 movement of a large number of individuals in a certain uniform direc- 



