BEHAVIOR OF THE LOWER ORGANISMS 



quence of the heat, a drop of cold water is placed on the upper surface 

 of the cover-glass. At once a dense collection of Paramecia is formed 



beneath it (Fig. 42). Observation of 

 the movements of the individuals 

 shows that this collection is formed in 

 ft the same way as the collections pro- 

 duced in chemicals (Figs. 43, 44, 

 etc.). The Paramecia at a distance 



FIG. 42. A slide of Paramecia is fr m the COoled region do not turn 

 heated to 40 or 45 degrees, then a drop of an( J swim directly toward it. But the 

 cold water (represented by the outline a) . . . . .. 



is placed on the upper surface of the Paramecia are swimming rapidly in 

 cover-glass. The animals collect beneath a \\ directions, and many enter every 



this drop, as shown in the figure. . ,, i .v ,v j 



instant the region beneath the drop. 



They do not react on entering, but on reaching the opposite side, where 

 they would pass out again into the heated water, they give the avoid- 

 ing reaction. This is repeated every time they come to the other 

 boundary of the drop, so that 

 the path of an individual within 

 the cooled region is similar to that 

 shown in Fig. 44. Every Para- 



mecium that enters the cooled 

 region therefore remains, and 



' 



soon a dense swarm is formed. 



A collection may be formed in 

 the same way by resting the slide * J r'i 



..' * . > drop of jJj per cent acetic acid. 



of Paramecia on a piece of ice and 



placing a drop of warmed water on the upper surface ; the Paramecia 

 now collect in the warmed region. But the collection is never so pro- 

 nounced as in the experiment last described, because the Paramecia 

 when cooled move less rapidly. 



Thus the Paramecia collect in certain regions because they give 

 the avoiding reaction when passing from certain conditions to others, 

 while when passing in the reverse direction they do not. Paramecia 

 at the normal temperature give the reaction at passing both to hotter 

 and to colder water ; they therefore tend to gather in water at the usual 

 temperature. This temperature at which they gather may be spoken 

 of as the optimum. Passage away from the optimum induces the 

 avoiding reaction ; passage toward the optimum does not. 



In the case of the chemicals thus far considered, the animals give 

 the reaction at passing from the weaker to the stronger solution, not at 

 passing in the opposite direction, so that they collect in the weaker solu- 

 tion. The optimum for these substances is thus zero, and this natu- 



