262 BEHAVIOR OF THE LOWER ORGANISMS 



5. In unicellular organisms, without a nervous system, certain parts 

 of the body may be more sensitive than the remainder, forming thus a 

 region comparable to a sense organ in a higher animal. Whether such 

 a part may become more sensitive to one form of stimulation, while 

 insensitive to others, as in higher organisms, seems not to have been 

 determined. 



6. Conduction occurs in organisms without a nervous system. This 

 is, of course, seen in the fact that a stimulus limited to one part of the 

 body may cause a contraction of the entire body, or a reversal of cilia 

 over the entire body surface. A strongly marked case is the contraction 

 of the stalk in Vorticella, when only the margin of the bell is stimulated. 



7. Summation of stimuli occurs in Protozoa as in Metazoa. This 

 is shown most clearly in Statkewitsch's experiments with induction 

 shocks (p. 83). Weak induction shocks have no effect until frequently 

 repeated. 



8. In the unicellular animal, as in that composed of many cells, the 

 reaction may change or become reversed as the intensity of the stimulus 

 increases, though the quality of the stimulus remains the same. Such 

 a change in reaction has sometimes been claimed as a specific property 

 of the nervous system. The protozoans Amoeba 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, 

 by other reactions. This has been shown in detail in the account of 

 Stentor (Chapter X). 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 invari- 

 able reflexes, depending only on the external stimulus and the anatomi- 

 cal structure of the organism. The reaction to a given stimulus de- 

 pends upon the physiological condition of the organism. In Stentor 

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

 acteristic reaction to the given stimulus. 



11. In unicellular as well as multicellular animals we find two chief 

 general classes of reactions, which may be designated positive and nega- 

 tive. 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 



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