182 REPRESENTATIVE SINGLE-CELLED ANIMALS 



stereotropism; a response to chemicals, chemotropism; and so on. 

 The word taxis is sometimes used in place of tropism, and thus we 

 have phototaxis, chemotaxis, etc. If one attempts to define a 

 tropism, not much more can be said, in view of the present disputed 

 use of the term, than that almost any response of an animal to a 

 stimulus may be called a tropism; but in practice the word is not 

 so often used for complex responses as for simple reactions. We 

 speak of the phototropism of a protozoan, of a moth flying into the 

 flame, even of a fish reacting to a fisherman's " flare "; but not so 

 commonly of the phototropism of dogs and other higher animals. 

 The important things to understand, in the present state of our 

 knowledge, are the visible manner in which each form responds 

 to a given stimulus, and, where a nervous system is present, the 

 internal processes. Any general theory of " tropisms " must 

 rest upon particular cases. There is still considerable disagree- 

 ment as to how these responses are brought about, whether, 

 for example, by trial and error or by a simpler means. 



In the case of paramoecium it therefore appears that response 

 to the kinds of stimulation which the organism meets in nature 

 occurs by means of a form of behavior known as the avoiding 

 reaction. This can be modified only within limits; but by per- 

 forming it a sufficient number of times, no matter how blindly, a 

 suitable adjustment to existing surroundings may be effected if any 

 such is possible. Thus the paramoecium gives evidence of the 

 same underlying phenomenon of irritdbility, or response to stimu- 

 lation, that is observed in amoeba and euglena and in higher 

 organisms. 



In view of the complexities of behavior in this species, it is of 

 interest to know that certain structures recentl}^ discovered in 

 Paramcpcium caudatum are interpreted as a miniature neuromotor 

 system, comparable in function with the nervous system of a 

 many-celled animal. There is an area near the anterior end of the 

 cytostome, called the motor center or motorium, from which minute 

 fibers extend to the peripheral parts of the cell (Fig. 98 A). The 

 ends of these fibers are described as connected with granules at 

 the bases of the cilia and with the trichocysts (Fig. 98 B). Con- 

 nected with the motorium are two lesser centers in the wall of the 

 cytopharynx (Fig. 98 C). Similar systems of more complicated 

 nature have been found in some other ciliates (Fig. 102, p. 194, and 

 Fig. 153, p. 315). If these systems are correctly interpreted as 



