12 THE BEHAVIOR OF LOWER ORGANISMS. 



slightest disturbance to the slide or trough containing the organisms. 

 The plan of this apparatus is taken from that of Mendelssohn. It can 

 be readily constructed in an hour or less, and gives essentially the same 

 results as Mendelssohn's more elaborate arrangement. With the use 

 of specially constructed thermometers, such as were employed by 

 Mendelssohn, exactly the same quantitative work could be done. The 

 present apparatus has the advantage that it is possible to place a 

 mirror beneath the glass slide or trough bearing the organisms, and 

 thus to observe the movements of the latter with the microscope by the 

 aid of reflected light. With the long-armed Braus-Driiner stand the 

 whole extent of the trough can be examined at ease, and the movements 

 of the organisms accurately observed with the stereoscopic binocular. 



As a trough I usually employed a glass slide, to which strips of glass 

 2 mm. in diameter had been cemented, making a trough 3 inches long, 

 about two-thirds of an inch wide, and 2 mm. deep. In some of the 

 experiments the trough was covered with a glass plate ; in others it 

 was left open. Both methods have their advantages and disadvantages. 



To realize the exact conditions under which the organisms are act- 

 ing it is necessary to consider a further question : What is the precise 

 nature of the stimulating agent in these experiments? Are we dealing 

 with radiant heat or with conducted heat? If we are dealing 

 primarily with radiant heat, of course currents in the water have 

 no effect on the distribution of the stimulating agent. If, on the other 

 hand, we are dealing with conducted heat, if the stimulating agent is 

 the heated or cooled water, then the conditions are different. Local 

 currents will cause local variations in the distribution of the heated 

 water. It is evident, I think, that the second alternative is in all 

 probability the correct one. Certainly in a bath-tub or in a long 

 vessel of any sort in which the water is heated at one end and not at 

 the other, it is possible by producing currents to vary the distribution 

 of the heated water and to perceive with the hand that it is this heated 

 water which acts as the stimulus. 



The importance of these considerations is evident when we take into 

 account the fact that the ciliate infusoria are always accompanied by 

 currents of typical character, having a definite relation to the form and 

 orientation of the animal's body. As a result of these currents, the in- 

 fusorian becomes not a mere passive recipient of stimulations, but an 

 active agent, determining by its activity how and in what part of the 

 body it shall be affected by stimuli. This may be illustrated by a 

 diagram (Fig. 6) showing the typical currents produced by the cilia 

 of Paramecium and the effect produced by these currents upon the 

 distribution of the heated (or cooled) water. The temperature is con- 



