STUDIES ON REACTIONS TO STIMULI. 231 



left. Whether on stimulation it turns toward a definite side could 

 not be determined. After a time the small piece fixed itself by 

 the foot and extended. The currents caused by the cilia were 

 most of the time directed away from the attached end, being 

 thus the reverse of the normal currents. At times, however, the 

 current passed toward the foot. 



When stimulated mechanically, the piece responded, like the 

 entire Stcntor, by bending into a new positiom, and later by con- 

 tracting. 



III. Spirostomum mnbignuui. Essentially the same results 

 were obtained with this large infusorian as with Stylonychia and 

 Stentor. Both the anterior half, bearing the adoral cilia, and the 

 posterior half, without adoral cilia, move and react in much the 

 same way as does the uninjured individual. They revolve to the 

 left when swimming freely through the water, and turn toward 

 the aboral side when stimulated. The anterior half moves more 

 rapidly than does the posterior half, and when stimulated turns 

 through a much greater angle than does the latter. The poste- 

 rior half, if not strongly stimulated, frequently remains nearly 

 quiet, or oscillates back and forth, as described in the third of 

 these studies (Jennings, 1899). If stimulated mechanically or 

 chemically, however, it backs and turns toward the aboral side. 



A posterior one third of the body reacts in the same manner 

 as the posterior one half. 



A minute piece from the posterior end, less than half the 

 length of the contractile vacuole, moved in the normal fashion, 

 revolving to the left, and moving with the anterior (cut) end in 

 front. The reactions to stimuli in such a small piece are very 

 difficult to determine. 



IV. Paramecium caudatum.--Paramecium is, of course, much 

 less favorable for work of this character than are the infusoria of 

 which an account is given above. This is due in part to the 

 minute size and rapid movements of Paramecium in part to the 

 strong tendency for the cut pieces to disintegrate at once. But, 

 with much patience, satisfactory anterior and posterior halves can 

 be obtained. 



i. Anterior half, bearing the oral groove and mouth (Fig. 4, a). 

 This moves and reacts almost exactly as does the entire animal, 



