82 THE BEHAVIOR OF LOWER ORGANISMS. 



continue this till death intervenes. In many cases they repeat the dart 

 forward and some escape in this way, while others do not. 



The reaction of Anuraea to heat is, therefore, not very precise, and 

 many individuals swim into the heated region and are killed. Those 

 which escape do so through a reaction which is similar to that of 

 those which do not ; in the one case the forward movement carries the 

 animal out of the heated region ; in the other it does not. The essential 

 point to the reaction is that the animals when stimulated by heat change 

 their course (through a " motor reflex"). This changed course nat- 

 urally is an advantage, and in accordance with the laws of probability 

 carries some of the organisms away from the source of danger. Others, 

 likewise in accordance with the laws of probability, are carried even 

 by the changed course toward the heated region, where they may be 

 killed unless a repetition of the "motor reflex" with its change of 

 course carries them finally away. The reaction is by the method 

 of" trial and error," and is not always successful. 



Altogether, the reaction of the rotifer Anursea to heat is of a charac- 

 ter similar in principle to that of Oxytricha (Fig. 7, p. 16). The 

 direction of turning depends on an internal factor ; the reaction takes 

 the form of " a motor reflex," and is by no means compatible with the 

 typical tropism schema. 



REACTION TO LIGHT. 



In light, as I have already set forth in the account of the reactions of 

 Stentor, we have a stimulating agent of a different character from that 

 found in chemicals or in heat, since the distribution of the stimulating 

 agent is not affected by the currents of water produced by the motor 

 organs of the animal. There is thus no reason in the distribution of 

 the stimulating agent to favor a turning toward one side rather than the 

 other. 



I have been able to study accurately the light reaction in but one 

 rotifer, Anurcea cochlearis Gosse. The conditions necessary for 

 precise observation of the nature of the reaction are very difficult to 

 fulfill, and the usual movements of the animals are such that the nature 

 of the reaction is obscured. As will be recalled, the organism is 

 normally swimming rapidly in a spiral, continually swerving toward 

 its dorsal side. This in itself is very confusing when one attempts to 

 observe just how the organism turns when stimulated. When light is 

 thrown upon it, or when the direction of light falling on it is changed, 

 the response is usually not given at once, and when it does occur, as 

 we shall see, it may be in the form of an accentuation of certain features 

 of the normal movement. From these conditions it results that it is 

 exceedingly difficult to tell, after a reaction to light has clearly occurred, 



