14 THE BEHAVIOR OF LOWER ORGANISMS. 



vortex of water passes as a slender stream along the oral groove of 

 Paramecium to the mouth. Consequently, water heated above the 

 threshold temperature reaches the Paramecium in this region before it 

 touches the body elsewhere. The result is thus a stimulation on the 

 oral side of the body, not elsewhere. 



Thus the way in which the organism is stimulated depends not ex- 

 clusively on the physical laws of the distribution of heat, but upon the 

 activity of the organism ; and the method of reaction, as we shall see, 

 is of a corresponding character. 



It is not difficult to observe the distribution of the currents above 

 described if one adds to the water on one side of the nearly or quite 

 quiet infusorian a cloud of very finely ground India ink. The same 

 results are obtained with other infusoria ; in Stentor, in Bursaria, and 

 in some of the larger Hypotricha the results are particularly striking. 

 Of course if the India ink, or the surface of threshold temperature, is 

 advancing obliquely to the axis of the infusoria, the results are more 

 complicated, and a diagram such as we have in Fig. 6 is not easy to 

 construct. But the result is uniformly to bring the stimulating agent 

 to the peristome before it reaches any other part of the body. It is not 

 possible to observe directly the distribution of water of different tem- 

 peratures, but under the influence of currents this of course follows, 

 essentially, the same laws as do fine particles suspended in the water. 



Another factor which it is important to take into consideration in 

 studying the effects of heat or other agents on the infusoria is the 

 greater sensitiveness of the anterior end and oral surface (or peristome) 

 as compared with the remainder of the body. This the present writer 

 has demonstrated for the anterior end by direct mechanical stimulation 

 in a considerable number of infusoria (Jennings, 1900), while Roesle 

 (1902) has shown a similar high comparative sensitivity for the peri- 

 stome region. The difference is such that in many cases where the 

 animal is completely enveloped by a stimulating agent (as by a chemi- 

 cal, or by warm or cold water) there is reason to think that the 

 reaction given is due to the stimulation at these regions alone. In 

 other words, the stimulus reaches its threshold value for the anterior 

 end and the region about the mouth much before it reaches this value 

 for the rest of the body. This consideration has an important bearing 

 on the theory which is frequently maintained, that the directive action 

 of a stimulus is due to the difference in its intensity on the two ends or 

 sides of the organism. Even if a stimulating agent acts, fer se, 

 slightly more strongly on the posterior end than on the anterior end 

 of an infusorian, there is reason to think that the reaction would be 

 conditioned entirely by the stimulus at the anterior end, this reaching 



