BULLETIN OE THE UNITED STATES FISH COMMISSION, 
296 
It is evident, therefore, that the general form of 
the body is adapted to Hie path which the body follows 
through the water. And this general form is produced 
by a twisting of the body from its original bilateral 
symmetry into the condition already minutely de- 
scribed. The reason why only the right half of the 
striated area is, as a rule, elevated into a ridge, which 
slopes to the right, and why the ridge lias an oblique 
course, are entirely evident in the light of the method 
of movement. The course is always a right spiral, 
and the single oblique ridge, sloping to the right, 
greatly favors the spiral movement, while if the left 
ridge were developed, it would act in opposition to 
the spiral course. The reason why the right side is 
concave, the left convex, with the consequent asym- 
metry of some of the internal organs (notably the 
trophi) is equally evident. All these things are adap- 
tations to the spiral movement, and, specifically, to 
movement in a right spiral. 
But there are some points which still need eluci- 
dation. Why lias the foot become twisted into such a 
position that the toes can be bent only to the right? 
Why does the right toe degenerate? And why are the 
teeth at the anterior dorsal margin of the lorica con- 
fined to the right side? 
These points will be better understood if we exam- 
ine the behavior a little further. As we have seen, 
the animals continually swerve, while swimming, 
toward the dorso-dextral part of the body — that which 
bears the ridge. This result is due to two components 
(1) a tendency to swerve toward the dorsal side, as 
when lifting the body from the bottom (a tendency 
which is present in almost all free-swimming rotifers), 
and (2) the revolution toward the right. The result- 
ant of these two components is a turning toward the 
dorso-dextral region. 
Now, as in the Infusoria/' the usual reaction to a 
stimulus in the Rattulidce is closely related to the 
method of locomotion. When a Diurella or Rattulus 
while swimming freely through the water meets an 
obstacle it alters its course simply by turning still 
farther than usual toward the side to which it is 
already swerving — that is, toward the dorso-dextral 
side. If the obstacle is small it is thus at once avoided. 
If the obstacle on the other hand is large, such as a 
fiat surface, which prevents further movement in the 
« See Jennings, On the Movements and Motor Reflexes of the Flagellata 
ancl Ciliata. Am. Journ. Physiol , vol. 3, pp. 229-260. 
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Fig. 2. — Spiral path followed by Diur- 
ella tigris Muller, showing that the 
animal continually swerves toward 
the dorso-dextral side. 
