22 THE BEHAVIOR OF LOWER ORGANISMS. 



Fixed specimens react less readily to heat than do free-swimming 

 specimens. They do not orient themselves with reference to the direc- 

 tion from which the rise in temperature comes. They may remain 

 extended normally, carrying on the usual activities, after the tempera- 

 ture has risen beyond the point which sets the free specimens in rapid 

 reaction. But as the temperature rises they repeatedly bend over into 

 a new position (bending toward the right aboral side), then contract 

 strongly, and finally free themselves from their attachment. There- 

 upon they behave like other free individuals. 



Spirostomum ambiguum : In this large ciliate the reactions to heat 

 and cold take place in essentially the same manner as is described 

 above for Stentor and the Hypotricha, so that it is not necessary to 

 describe the phenomena in detail. The organism reacts to heat or 

 cold by backing and turning toward its aboral side ; and this whether 

 the change in temperature is uniform over the entire surface of the 

 animal or whether it approaches from one side. The movements of the 

 animal are slow, and under the Braus-Driiner stereoscopic microscope 

 its method of reaction is very clear. There is little marked common 

 orientation at any time, however ; this being due to the slowness of the 

 movements and the frequency of repetitions of the motor reaction. 



Bursaria truncatella: In this very large infusorian, in which cer- 

 tain differentiations of the body are visible even to the naked eye, the 

 method of reaction to heat and cold is observed with the greatest ease. 

 But orientation of a large number of individuals in a common direction 

 is hardly to be noticed, though if Bursaria could be obtained in such 

 numbers as Paramecium or Oxytricha, perhaps an indication of orien- 

 tation would be noticeable in spite of the slowness of movement. 



Bursaria is very inactive, often remaining quiet for long periods. 

 It swims slowly, and frequently creeps along the bottom with ventral 

 side down, but may also swim freely through the water, revolving to 

 the left. If the temperature of the trough is raised at one end, the 

 animals in this region that are moving freely through the water swim 

 backward, turn to the right, and swim forward. This may be repeated 

 till the organism passes out of the heated region. Rather more 

 frequently, however, the animal, after thus reacting once or twice, sinks 

 to the bottom and places its ventral side against the surface. It now 

 conducts itself in the same manner as do the other individuals in this 

 situation, as will be described. 



The individuals which are resting against the bottom (usually the 

 majority of those in the trough) react as follows : They begin to swim 

 backward, keeping the ventral side down and at the same time circling 

 toward their own right sides. They thus describe rather narrow circles. 



