No. 389] STIMULI IN UNICELLULAR ORGANISMS. 377 
cept that the animal swims only a short distance backward 
before turning. 
2. Stimulus at Posterior End.— If now the animal is touched 
at the posterior end, exactly the same reaction is produced. 
It contracts, then swims backward, — therefore toward the point 
of stimulus (whereas in the other case it swam away from the 
point of stimulus). Then it curves toward the aboral side, 
describing a wide spiral, and finally swims forward in the 
direction of the aboral side, exactly as described above. It 
makes no difference to the reaction which way the animal is 
swimming when stimulated :. if swimming forward, the direction 
is changed ; if swimming backward when touched at the pos- 
terior end, it continues to swim backward after contracting. 
The posterior end is slightly less sensitive than the anterior, 
so that a very weak stimulus at the posterior end may cause no 
reaction at all. No matter where stimulated there are rare 
cases in which the animal when stimulated merely contracts 
(always darting back a little at the moment of contraction), then 
stops for a time, then resumes a forward motion. Out of one 
hundred cases stimulated at the anterior end, ninety-six reacted 
in the typical manner, while four gave the incomplete reaction 
just mentioned. Out of one hundred cases stimulated at the 
posterior end, ninety-two gave the typical reaction, while eight 
reacted incompletely. This difference in the number giving 
the typical reaction is probably a mere statistical variation, 
which would disappear with larger numbers; if any significance 
is to be attached to it, it is merely that already noted — that the 
posterior end is slightly less sensitive than the anterior. 
The reaction is thus exactly the same whether the stimulus 
occurs at the anterior or the posterior end. 
3. Stimulus at One Side. — When the animal is touched any- 
where on the surface of the body between the two ends, the 
same reaction is given as in the two foregoing cases; the ani- 
mal contracts, swims backward, curves toward the aboral side, 
and swims forward toward that side. The final direction in 
which it swims bears no relation to the position of the side 
on which the stimulus was given. 
4. Stimulus Unlocalized. — An unlocalized mechanical stimu- 
