PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 85 



may affect the nervous system or sense organs may likewise affect pro- 

 toplasm without these organs. Even the naked protoplasm of araeba 

 responds to all classes of stimuli to which any animal responds. The 

 nervous system and sense organs are, therefore, not necessary for the 

 reception of any particular classes of stimulations. 



2. "The reactions produced in unicellular organisms by stimuli are 

 not the direct physical or chemical effects of the agents acting upon 

 them, but are indirect reactions, produced through the release of 

 certain forces already present in the organism. In this respect the 

 reactions are comparable with those of higher animals. It is true for 

 ameba as well as for more differentiated protozoa. 



3. " In the protozoa, as in the metazoa, the structure of the organism 

 plays a large part in determining the nature of the behavior. There 

 are only certain acts which the organism can perform, and these are 

 conditioned by its organization; by one of these acts it must respond 

 to any stimulus. If the behavior of the metazoa is comparable in this 

 respect to the action of a machine, the same comparison can be made 

 for the behavior of the protozoa. 



4. "Spontaneous action — that is, activity and changes in activity 

 induced without external stimulation — takes place in the protozoa as 

 well as in the metazoa. Both vorticella and hydra, as we have seen, 

 spontaneously contract at rather regular intervals, even when the 

 external conditions remain uniform. Continued activity is the normal 

 state of affairs in paramecium and most other infusoria. The idea 

 that spontaneous activity is found only in higher animals is a totally 

 erroneous one; action is as spontaneous in the protozoa as in man. 



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 con- 

 traction 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. Weak hiduction shocks have no effect until frecjuently 

 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 



