REACTIONS TO HEAT AND COLD. 15 



its threshold value before the stimulus elsewhere produces any effect. 

 Corresponding statements could be made with relation to the oral and 

 aboral sides. Of course, owing to the course of the currents above 

 described, any stimulating agent whose distribution is affected by cur- 

 rents in the water will usually reach the anterior end and oral side 

 first in any case. 



Summing up, we find (i) that the threshold intensity of a stimulating 

 agent whose distribution is affected by currents in the water will reach 

 the anterior end and oral side of the organism before it reaches other 

 parts of the body ; (2) that the anterior end and oral surface are more 

 sensitive than the rest of the body, so that the threshold value for 

 stimuli is less here than elsewhere. 



We may now proceed to an account of the observed method by 

 which some of the organisms react to heat and cold. 



Oxytricha fallax : This is one of the most favorable of the Ciliata 

 for determining the method of reactions to stimuli, for two reasons. 



(1) It is easily procurable in large numbers, occurring in cultures of 

 the same sort that produce Paramecium, and in equal abundance. 



(2) It does not, as a rule, revolve rapidly on its long axis, as Para- 

 mecium does, but usually creeps with its oral or ventral side against a 

 surface, so that it is not difficult to observe the relation of the reaction 

 movements to the differences in the sides of the body. 



When water containing a large number of Oxytrichas is placed in 

 the trough and one end of the trough is heated by passing warm water 

 through the tube which underlies it, the Oxytrichas gradually pass 

 toward the opposite end of the trough, forming a dense assemblage 

 with a rather sharply defined edge toward the heated side. If the end 

 at first heated is now cooled and the opposite end heated, the organisms 

 pass back to the end from which they first came. Similar results are 

 obtained by making one end very cold ; the animals gather in an 

 optimum region, avoiding both too great heat and too great cold.* 

 The phenomena are identical with what is to be observed in the case 

 of Paramecium, save that it requires somewhat longer for the Oxytrichas 

 to move from one end of the trough to the other, and the progress in a 

 definite direction is not so steady as we find it in Paramecium. 



If the movements of the individuals are observed we find them to be 

 as follows : Near that end of the trough where the temperature is 



* Many quantitative data for various infusoria are given in the valuable papers 

 of Mendelssohn. As the object of the present paper was not to obtain quantita- 

 tive data, but to determine just how the animals acted, absolute temperatures 

 are not recorded. In every case the experiments were so varied as to use at 

 times temperatures to which a reaction was hardly noticeable; at other times 

 more extreme temperatures, up to those which were destructive. 



