LIFE HISTORY OF TWO RARE CILIATES 395 



or alcohol; that whole specimens were eaten while a jelly com- 

 posed of crushed Paramoecia and Euglena was rejected. As 

 a logical deduction from these results Schaeffer concluded that 

 Stentor does exercise a selection among particles which are 

 brought into the food pouch; it discriminates between digestible 

 and indigestible particles and between different kinds of organ- 

 isms. What then is the basis of this choice? Making a dis- 

 tinction between taste, a reaction to chemicals, and touch, a 

 reaction to form, he concludes that inasmuch as Stentor ate 

 alike living forms and those fixed in chemicals while it chose 

 entire organisms in preference to macerated ones, that the selec- 

 tion must be made on a tactual basis. 



Didinium, a predatory ciliate, appears to the casual observer 

 to exercise a somewhat limited choice. Jennings, however, 

 explains the food habits of this form by the trial-error theory, 

 claiming that Didinium reacts not only to particles which may 

 serve as food but to all kinds of solid bodies, that it is constantly 

 coming into contact with various substances digestible and indi- 

 gestible, each one of \^ hich it 'tries' to pierce and swallow. If 

 successful the particle is chosen as food, if the trial be unsuccess- 

 ful it is rejected and classed with the errors. Jennings says 

 ''there is no evidence that in some unknown way the infusoria 

 perceive their prey at a distance nor that they decide beforehand 

 to attack certain objects and leave others unattacked. They 

 simply prove all things and hold fast to that which is good." 

 In this conclusion Jennings agrees with Maupas. 



It is evident, however, that the trial-error explanation is 

 inadequate in dealing with the behavior of Actinobolus and 

 Spathidium. It is true that when the organisms are in a con- 

 dition of satiety, the small ciliates Halteria and Colpidium may 

 pass their enemies unharmed; if, on the other hand, Actinobolus 

 and Spathidium are hungry, no attempt is made to eat any other 

 kind of food than the accustomed prey. We find here no 'trials' 

 but a definite selection of food material. In experimentation 

 with unicellular forms under identical conditions not only has 

 a difference in reaction between different individuals been noted, 

 but also difference in reaction of the same individual at differ- 



