ROTATORIA OF THE UNITED STATES. 
297 
a 
d 
axis of the spiral, the animal continues to swerve toward the dorso-dextral side till 
its general direction is completely changed. Text figure 3 represents such a reac- 
tion in Diurella tigris Muller. 
The animal may he stimu- 
lated in other ways; the usual 
result is to induce it to swerve 
farther toward the dorso-dextral 
side. If there is really no obsta- 
cle the path becomes merely a 
wider spiral than usual for some 
distance. 
Now, it is evident that if 
the animal, when thus turning, 
strikes against any object, it 
will be the dorso-dextral angle 
of the head which receives the 
shock. The corona is of course 
not covered by the lorica, as is the rest of the body, 
so that it might easily be injured in such cases. But 
at the point where the corona would strike — at the 
dorso-dextral angle — is the tooth (or teeth). This 
takes the blow which would otherwise fall upon the 
delicate corona. 
Sometimes the animal swims forward into a small 
angle, where it can not directly turn, as between the 
surface film of water and the bottom of a watch glass. 
In this case the animal begins, as usual, to turn toward 
the dorso-dextral side, but as a result it may merely 
“bump” its head against the bottom. It neverthe- 
less perseveres trying to turn in the same direction, 
while at the same time it revolves on its long axis. 
Thus the head will be dragged and “bumped” along 
the surface until in time the dorso-dextral angle 
(through the revolution) becomes directed toward the 
free water. No one who has seen this peculiar per- 
formance (which is not at all uncommon) can remain 
in doubt as to what is the significance of the tooth or 
teeth at the dorso-dextral side of the anterior end of 
the lorica. These teeth take all the “bumping” 
while the animal is turning, in place of its falling 
upon the delicate corona. The teeth are placed just 
where they will serve to protect the delicate head 
when the anterior end comes in contact with any- 
thing. Owing to the invariable swerving toward the 
dorso-dextral side, the head, if it ever strikes against obstacles at all, will strike on 
this dorso-dextral angle, where the teeth are ready to protect it. 
The striking against objects is by no means rare even in the ordinary swimming 
Fig. 3. — Diagram of a reaction to a stim_ 
ulus in Diurella tigris Mtiller. A rep- 
resents an obstacle. The animal turns 
toward the dorso-dextral side, which 
bears the. tooth and ridge. 
