THE PROTOZOA OF LAKE ERIE. 
107 
same is true for these. Spirostomum ambiguum when stimulated contracts, swims back- 
ward, turns toward the aboral side, and swims forward. If touched with a spicule of 
glass at its anterior end it swims backward, away from the glass; if touched at the 
posterior end it likewise swims backward, therefore toward the glass. Stentor poly- 
morphus when stimulated contracts, swims backward, turns to the right, then swims 
forward, the direction of motion having, as in the other cases, no relation to localization 
of the stimulus. Similar results, not yet published in detail, have been obtained with 
certain Flagellata, as well as with other Ciliata.* 
Besides determining the general mechanism of the reactions of Paramecium, au 
exlended study was made of the effects upon its activities of different chemicals. If 
into a preparation of Paramecia mounted upon the slide a drop of some chemical 
substance in solution is introduced, the Paramecia may either collect in the drop or 
leave it entirely empty. In the former case they show positive chemotaxis to the 
substance; in the latter case, negative chemotaxis. As to what might be called the 
mechanism of chemotaxis, the following was made out. The Paramecia are not 
attracted by the drop of substance into which they gather. They may graze the very 
edge of the drop without swerving a particle from their course so as to paiSS into it. 
But Paramecia when first brought upon the .slide swim rapidly in every direction, so 
that many will quickly come by chance against the edge of such a drop. They do not 
react, but swim straight ahead — the substance in the drop not acting as a stimulus to 
produce the motor reaction above described. But on passing across the drop, the 
outer fluid does, after the Paramecia have been in the drop of the chemical, act as a 
stimulus to produce the motor reaction. The Paramecium therefore, on coming to 
the outer edge of -the drop, swims backward, thus returning into the drop. It then 
turns toward the aboral side and swims forward (in accordance with the general 
scheme of reaction above described). 1 1 thus comes to the outer edge of the drop once 
more; reacts again, and this being kept up, is continually prevented from crossing the 
boundary of the drop into the surrounding water. The same process is repeated for 
many Paramecia, until in time the drop swarms with the Infusoria, so that they 
appear to have been attracted into it. 
In case of a substance in which the Paramecia do not collect at all, observation 
shows that the motor reaction (swimming backward, turning, and swimming forward) 
is produced when the Paramecia come against the drop from the outside; hence they 
are prevented from entering and the drop remains empty. 
It was found possible to classify chemical compounds thus into two classes. On 
the one hand may be placed together those which do not produce the reaction when 
the Paramecia enter them, but throw the animals into such a physiological condition 
that they do react when they pass out of a drop containing the substance in question. 
In these the Paramecia, therefore, gather and are said to be positively ehemotactic 
toward them. On the other hand are substances which produce the motor reaction as 
soon as the Paramecia come in contact with them, so that the animals do not enter 
* The reactions of a targe number of Protozoa haVe been studied since the above was written. In 
all of these the direction of turning was found to be determined entirely by internal factors, and to 
have no relation to the position of the source of stimulus. The direction of motion along the body 
axis, on the other hand, was found in a number of cases, for mechanical stimuli, to bo determined by 
the localization of the stimulus. Loxodes rostrum, for example, when touched with a glass rod at the 
anterior end swims backward; touched at the posterior end it swims forward. For chemical stimuli, 
however, the absence of any such dependence of the direction of movement on the localization of the 
stimulus was demonstrated. For details, see a paper by the author on “ The movements and motor 
reflexes of the Flagellata and Ciliata,” in the American Journal of Physiology, January, 1900. 
