The Mind of the Simplest Animals 53 



the contact reaction is more likely to occur if the animal 

 comes against the solid when swimming rather slowly. Jen- 

 nings reports also that individuals vary. " Often all the indi- 

 viduals in a culture are thus inclined to come to rest, while 

 in another culture all remain free-swimming, 

 and give the avoiding reaction whenever they 

 come in contact with a solid " (211, p. 60). This 

 would suggest that some individuals are in a state 

 of greater excitability than others, so that a given 

 stimulus acts more strongly upon them. On the 

 other hand, there is a possibility that qualitative 

 as well as intensive differences in the stimulus 

 are responsible for the contrasting reactions. 

 "In general," says Jennings, Paramecium 

 "shows a tendency to come to rest against loose 

 or fibrous material; in other words, it reacts 

 thus to material with which it can come in 

 contact at two or more parts of the body at once. 

 To smooth, hard materials, such as glass, it is much less 

 likely to react in this manner " (211, p. *6i). Perhaps, then, 

 the spatial distribution of the stimulus over several points 

 of the body surface increases the probability of a contact 

 rather than an avoiding reaction. 



Certain other forms of behavior in Paramecium involve 

 the taking up of a definite position with reference to some 

 constant stimulus, and are therefore termed by Jennings 

 "orienting reactions." In the first place, if there is a current 

 in the water, the animals will head up-stream. Jennings 

 explains this as due to the giving of avoiding reactions in 

 response to the disturbing effects of the current on the cilia 

 until, with the Paramecium's head up-stream, the current no 

 longer tends to reverse the cilia. Analogous reaction is given 

 to gravity ; the animals direct their heads upward, and swim 



FIG. 5. 

 Positive 

 thigmotaxis 

 in Parame- 

 cium. After 

 Jennings 

 (211). 



