COMPARISON OF PROTOZOA AND METAZOA 261 



fundamental features of behavior. Do we find a striking difference in 

 the behavior of organisms after a nervous system has been developed ? 

 What can animals do without a nervous system? A comparison of 

 organisms with and those without this system should give us evidence as 

 to the real nature of the functions of the latter, and will perhaps 

 prevent us from overestimating its importance. 



We will sum up briefly in a number of paragraphs the resemblances 

 and differences between the behavior of animals with and without a 

 nervous system. 



I. First, we find that in organisms consisting of but a single cell, 

 and having no nervous system, the behavior is regulated by all the dif- 

 ferent classes of conditions which regulate the behavior of higher animals. 

 In other words, unicellular organisms react to all classes of stimuli to 

 which higher animals react.^ All classes of stimuli which may affect 

 the nervous system or sense organs may likewise affect protoplasm 

 without these organs. Even the naked protoplasm of Amoeba 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 com- 

 parable with those of higher animals. This is true for Amoeba 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 condi- 

 tioned 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. 



04. 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 spon- 

 taneous activity is found only in higher animals is a totally erroneous 

 one ; action is as spontaneous in the Protozoa as in man. 



1 Considering auditory stimulation as merely a special case of mechanical stimulation. 



