386 THE AMERICAN NATURALIST.  [Vot. XXXIII. 
right, and swims forward on a new path; if this path takes it 
again into the stimulating region, the reaction is repeated ; 
thus by the laws of chance the animal will in time probably 
be brought into a region which does not act as a stimulus. 
The reaction is the same whether the stimulus is localized or 
is a general one. The reaction is in essentials like that of 
Paramecium and Stentor. 
The reaction of Stentor polymorphus is shown in Fig. 4. 
SUMMARY AND CONCLUSIONS. 
The reactions of Spirostomum and Stentor are similar in all 
essentials to those of Paramecium. To any stimulus all these 
animals respond by swimming backward, turning to one side, 
and then swimming forward. Paramecium and Spirostomum 
always turn toward the aboral side; Stentor, toward the right 
side. In Spirostomum and Stentor a contraction of the body 
forms an additional feature of the reaction. The reaction is 
not modified in any way by the position of the stimulus; the 
direction of motion is the same whether the source of stimulus 
is at the anterior end, the posterior end, the side, or if the 
stimulus is not localized at all. If the stimulus is at the 
anterior end, the animal necessarily swims away from it; if at 
the posterior end, it swims toward it, even when this results 
in the destruction of the animal. The discussion and conclu- 
sions given in my previous paper (loc. cit.) in regard to Parame- 
cium are, therefore, equally applicable to Spirostomum and 
Stentor, and need not be repeated here. 
The fact that three such dissimilar ciliates as Paramecium, 
Spirostomum, and Stentor agree in their reaction in all essen- 
tial particulars certainly raises a presumption that the mode of 
reaction is of this general character throughout the ciliate Infu- 
soria. This is especially probable in view of the fact that the 
revolution on the long axis and progress in a spiral course, 
which plays so essential a part in these reactions, and indeed 
seems to have the special purpose of making such a method of 
reaction possible, is already known to occur in the motions of 
almost all ciliates. Moreover, the same is known, through the 
