20 THE BIOLOGY OF STENTOR 



this avoiding reaction in roeseli and coeruleus, prodding them with 

 a glass needle or, by gentle use of a pipette, injecting into the oral 

 vortex substances of weak chemical stimulation, like carmine 

 particles. As if to conserve their status quo, the animals performed 

 a series of distinctly different responses of increasing intensity 

 until the stimulus was avoided (Fig. 2b). 



First, a light touch as from an impinging rotifer which could 

 serve as food provoked no response in coeruleus, which merely 

 went on feeding, and roeseli even made the positive response of 

 bending toward the source of stimulation. 



When the stimulus was a little stronger, as from a large, hard 

 object or a cloud of carmine particles, the stentors '* turned away." 

 Remaining attached but twisting one or two turns on the axis, the 

 animals then bent toward the aboral side always and therefore not 

 necessarily away from the source of stimulation. This reaction is 

 quite like the avoiding response of Paramecium: a fixed response 

 without reference to the direction of stimulus, repeated if un- 

 successful. 



Third, the membranelles might stop and body cilia reverse for 

 an instant, thereby propelling the carmine particles forward and 

 away from the anterior end. Feeding currents then continued and 

 the reaction was repeated several times if the particles were still 

 encountered. This response sometimes occurred instead of turning 

 away, but the variability may have been due to the difficulty of 

 providing precisely graded stimuli. 



If the noxious chemical stimulus still persisted, or if poked with 

 the needle, the stentors instantly contracted, slowly extended again, 

 and re-contracted if conditions were still undesirable. This reaction 

 could continue for fifteen minutes if carmine particles were kept 

 available or prodding repeated. 



Fifth and finally, the holdfast was set free and the stentors swam 

 away seeking a new environment. Sometimes co^rw/^w^ detached and 

 swam away after the first stimulus but usually the other avoiding 

 responses intervened. From my own observations it appears that 

 stentors from cultures which have recently been fed are more 

 likely to persist in the feeding response and to give the graded 

 response, as if bothered by the interruption of a good thing, 

 whereas unfed animals are more likely to detach and swim away 

 at once, as if the negative stimulus finally prodded them to 



