86 PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 



same. Such a change in reaction has sometimes been claimed as a 

 specific property of the nervous system. The protozoa ameba and 

 stentor, as well as the metazoan planaria, move toward sources of 

 weak mechanical stimulation, away from sources of strong stimulation. 



9. "In the protozoa, as in the metazoa, the reaction may change 

 while the stimulus remains the same; that is, the animal may respond 

 at first by a certain reaction; later, while the stimulus remains the 

 same, bv other reactions. This has been shown in detail in the account 

 of stentor. The change may consist in either a cessation of the reaction 

 or in a complete alteration of its character. These changes are, as a 

 rule, by no means due to fatigue, but are regulatory in character. The 

 behavior thus depends on the past history of the organism. For such 

 modifications of behavior a nervous system is then unnecessary. 



10. "In the protozoa, as in the metazoa, the reactions are not 

 invariably reflexes, depending only on the external stimulus and the 

 anatomical structure of the organism. The reaction to a given stimulus 

 depends upon the physiological condition of the organism. In stentor 

 we could distinguish at least five different conditions, each with its 

 characteristic reaction to the given stimulus. 



11. "In unicellular, as well as in multicellular, animals we find two 

 chief general classes of reactions, which may be designated as positive 

 and negative. The positive reaction tends to retain the organism in 

 contact with the stimulus, the negative to remove it from the stimulus. 

 In many classes of stimuli we can distinguish an optimum condition. 

 A change leading from the optimum produces a negative reaction, 

 while a change leading toward the optimum produces no reaction, or a 

 positive one. The optimum from this standpoint usually corresponds, 

 in a broad way, to the optimum for the general interests of the organ- 

 ism. These relations hold equally for protozoa and metazoa. 



12. "In both the protozoa and the metazoa that we have studied, 

 the behavior is based to a considerable degree on a selection of certain 

 conditions through the production under stimulation of varied move- 

 ments. When the organism is subjected to an irritating condition, it 

 tries many ditterent conditions or many dift'erent ways of ridding itself 

 of this condition, until one is found which is successful. 



"Altogether, there is no evidence of the existence of differences of 

 fundamental character between the behavior of the protozoa and that 

 of the lower metazoa. The study of l)ehavior lends no support to the 

 view that the life activities are of essentially different character in the 

 protozoa and metazoa. The behavior of the protozoa appears to be 

 no more and no less machine-like than that of the metazoa; similar 

 priiioi])les govern both." 



Growth and Reproduction.— In all of the constructive pro- 

 cesses of the cell there is no doubt that the nucleus plays the most 

 important part, and that it is, in a sense, the directive centre of activi- 



