THE BEHAVIOR OF INFUSORIA; PARAMECIUM 55 



in any way, but swim directly into it. They continue across till they 

 reach the farther boundary, where they come in contact again with the 

 \ per cent solution. Here the reaction occurs. The animals give 

 the avoiding reaction, swimming backward, turning toward the aboral 

 side, and starting forward again, etc. They of course soon come in 

 contact again with the outlying -|- per cent NaCl, whereupon they react 

 as before, and this continues, so that they do not leave the drop of jq per 

 cent NaCl. The path of a single Paramecium in such a drop is like 

 that shown in Fig. 44. Since all the infusoria that enter the drop of 

 Yq per cent NaCl remain, it soon swarms with them. 



In place of NaCl, we may use pairs of solutions of other chemicals, 

 one stronger than the other, — taking pains of course not to employ 

 concentrations that are decidedly injurious. With any of the ordinary 

 inorganic salts or alkalies the animals collect in the weaker solution, 

 through the fact that they avoid the stronger one in the way described 

 above. The same concentration of a given chemical may play opposite 

 roles in successive experiments, depending on whether it is associated 

 with a weaker or a stronger solution. In the former case the Paramecia 

 avoid it ; in the latter they gather within it. If the weaker solution sur- 

 rounds a drop of the stronger, the latter is left empty, and the Para- 

 mecia remain scattered through the preparation, as in Fig. 41. If 

 the stronger solution surrounds the weaker, the latter becomes filled 

 with the Paramecia, as in Fig. 43, while the former is left nearly 

 empty. Thus with the same pair of substances we get either a dense 

 aggregation (or what is often called positive chemotaxis), or a certain 

 area left vacant ("negative chemotaxis"), depending on the relation 

 of the two fluids to each other. 



If we use pure water in place of the weaker solution, we get the same 

 result ; the Paramecia collect in the drop of water. This is easily shown 

 by introducing a drop of water into a preparation of Paramecia that have 

 been mixed with ^ per cent NaCl ; the water soon swarms with the in- 

 fusoria. The culture water in which Paramecia live usually contains 

 various salts, and is often alkaline in reaction. If a drop of distilled 

 water is added (as in Fig. 40) to a preparation of infusoria in such 

 culture water, the animals gather in the distilled water. 



The same results may be obtained with water of differing tempera- 

 tures. This is done by surrounding an area of water at the normal 

 temperature with water at a temperature considerably higher or lower. 

 The Paramecia may be placed on a slide in the usual way, with a cover- 

 glass supported by glass rods. This slide is then placed on a bottle 

 or other vessel containing water heated to forty-five or fifty degrees. 

 As soon as the Paramecia begin to move about more rapidly in conse- 



