440 GENERAL BIOLOGY 



The specialized activities of Paramoecium have made of it 

 a good deal of an automaton. Aside from the ordinary 

 swimming, in a spiral course, with the oral groove toward 

 the axis of the spiral (see page 73), it has one reaction that 

 is prominent above all others a definite avoiding reaction. 

 Meeting with almost any sort of stimulus it stops, ceases 

 rotation, swims backward a little way, turns a little to the 

 aboral side, and then starts swimming forward again (fig. 

 2570). If the change of direction were not sufficient to 

 clear the obstruction (or other source of stimulus), the 

 reaction is repeated; and it continues to be repeated in a 

 perfectly automatic manner until the course is again clear. 



Food-taking in paramcecium appears, as we have seen 

 (page 75), to be an equally automatic and undiscriminating 

 performance, digestible and indigestible substances being 

 taken into the body with equal, readiness. 



There is also a contact reaction observable in Paramce- 

 cium. When, in swimming about, one comes into gentle 

 contact with a bit of soft vegetable substance, it may come 

 to rest in the manner shown in figure 2576, with a number of 

 its cilia extended in direct contact with the object. Thus, 

 it may remain quietly, as if at a satisfactory anchorage, for a 

 considerable time. Doubtless it is in the neighborhood of 

 such soft substances that its proper food is likely to be found. 

 And when a paramoecium is thus resting it is less responsive 

 than at other times to ordinary stimuli. For example, a 

 slight mechanical stimulus, such as a light touch on the tips 

 of its cilia, may pass unnoticed, whereas the same stimulus 

 if applied to one not thus situated would instantly call forth 

 the avoiding reaction. 



In all the Protozoa there are certain activities (and cessa- 

 tions of activity, equally significant) that indicate that the 

 actuating stimuli are of internal origin. Apparent spon- 

 taneity of movement may be due to external influences that 



