174 BIOLOGY OF THE PROTOZOA 



compatiljle: the former conveys the idea of a directive stimu- 

 lation upon local motor organs or controlling elements; the latter 

 implies the complex reaction of a definite mechanism charac- 

 teristic of any specific protoplasm, and the same reaction follows 

 upon stimulation by any type of stimulus (Piitter, 1903, Jennings, 

 1909). It follows further that the reaction is called forth regardless 

 of the particular organ or element first to receive the stimulus. 



We owe Jennings the credit for first clearly distinguishing between 

 these two conceptions, as well as for careful analyses of the move- 

 ments of lower organisms (1904 et. seq.), and for demonstrating the 

 particular motor response distinctive of specific tv'pes of Protozoa. 

 He also showed that the nature of the motor response in some 

 organisms, e. f/., in Stentor, is correlated with the physiological 

 state of the organism, and adduced evidence which indicates that 

 phenomena of fatigue are involved. The classical example of a 

 motor response, formerly interpreted as chemiotaxis, is the case of 

 Paramecinm caudaiuvi or avrelia in a drop of dilute acid. Casual 

 swimming brings the individual to the outer limit of the drop; the 

 transition from water to drop does not provide a. stimulus strong 

 enough to bring about the motor response and the individual con- 

 tinues through the drop until it strikes the farther limit. Here 

 the stimulus is sufficiently strong to cause the motor response wdiich 

 is manifested as a backward swimming, due to reversal of cilia, 

 turning on the long axis and recovery of normal forward swimming 

 movement. Repetition of this procedure keeps the individual in 

 the acid drop. Others enter in a similar way and are similarly 

 trapped until many are confined in the acid drop where the}" are 

 ultimately killed. Such motor responses unquestionably play an 

 important role in food-getting and in vital activities generally. 



The stereotyped nature of the motor response in any specific 

 organism may be due in the main to the characteristic neuromotor 

 systems which the higher types of flagellates and ciliates possess. 

 The observations of Sharp "(1914), Yocom (1916) and McDonald 

 (1922) on ciliates, of Kofoid on flagellates, and the experiments of 

 Taylor (1920) in cutting different regions of the neuromotor complex 

 of FAiylotes, indicate that the motor response of Protozoa is bound 

 up with coordinating systems possessing some of the attributes of 

 coordinating systems in Metazoa (Fig. 86). Knowledge of these 

 complex systems and their reactions is quite sufficient to dispel 

 any lingering belief in tropisms as due to stimulation of special 

 motile elements acting independently in such a way as to orient 

 the organism in respect to the path of the stimulus. Through 

 coordinating fibrils all parts work together; cutting the system at 

 any point leads to inharmonious or uncoordinated movements of 

 the motile organs as Ta^dor has demonstrated. All reactions depend 

 upon the organism as a whole; enucleated fragments are unable to 



