ROTATORIA OF THE UNITED STATES. 
295 
very justly. Anterior and posterior ends differ because they come into different 
relations with the environment, owing to the forward movement. In the same way 
dorsal and ventral surfaces differ because they come into different relations with the 
environment — the ventral side being more commonly in contact with a surface, the 
dorsal side not thus in contact, but subjected to the light and other influences coming 
from above. On the other hand, the right and left sides are in a similar relation to 
the environment, there being no influence which acts on one differently from the 
way it affects the other; lienee they remain alike. 
Analogous considerations apply to the radially symmetrical form. But there is 
another type of structure, having an equally definite relation to the method of life 
and movement — a type which has not been hitherto recognized, at least not as having 
a definite relation to a widespread method of locomotion and life. This is what may, 
in general, be characterized as a spiral type of structure, or at least as a one-sided 
type. This type of structure is found in many organisms which swim freely through 
the water in a spiral course. Its typical representatives are the Infusoria — the 
Ciliata and Flagellata. 
The spiral course may be characterized as the simplest device to enable an 
organism to make progress in a given direction through the free water without ful- 
filling the difficult condition of making all sides identically alike, or of making the 
differences exactly balance each other.® In the spiral course the organism continually 
keeps one side toward the outside of the spiral. In other words, it is in reality 
always turning toward one side. The tendency to deviate thus caused is compen- 
sated by a revolution on the long axis, which continually brings the side in question 
into a new position. The path thus becomes a spiral, while if revolution on the long 
axis did not occur it would be a circle. 
Now, the organisms which habitually make use of this method of progression 
have a form which is adapted to it. In the ciliate and flagellate Infusoria, which 
move in this manner, the form is usually unsymmetrical, often clearly spiral; and 
here the spiral form seems to be primitive; at least it was not developed from an 
originally bilateral form. But in the Rattulidce we have a group of animals, 
fundamentally bilateral, which are taking on this spiral, unsymmetrical form as an 
adaptation to their method of movement. 
Movements of the Rattulidce . — If we examine in detail the movements of one of 
the Rattulidce , taking, for example, Diurella tigris Muller (fig. 1), we find that it 
swims through the wafer in a spiral, of such a course that its twisted body forms 
a segment of the spiral path (text figure 2). The animal revolves to the right and 
swerves toward its dorso-dextral side, while it at the same time progresses. The 
result is a path almost exactly that which would be produced if the animal were 
moving on the inside of a hollow cylinder and the dorso-lateral spiral ridge ran in a 
groove on the inner surface of the cylinder, which fitted it precisely and had the 
same curvature. The effect is the same as that produced by the spiral grooves on 
the inner surface of a rifle barrel, giving the ball a rotary motion about the axis of 
flight. The result is here, as in the rifle ball, to make the axis of progression a 
straight line. 
“For the grounds on which this statement is based, as well as a general discussion of spiral movement and 
unsymmetrical structure, see the paper on Asymmetry , etc., already cited (p. 293); also a paper by the present author 
on The Significance of the Spiral Sivimming of Organisms , in the American Naturalist, vol. 35, 1901, pp. 369-378. 
