AND ITS BIOLOGICAL SIGNIFICANCE. 



331 



FIG. I. Spiral Path followed by Diu- 

 rellatigris Miillerwhcn freely swim- 

 ming. It will be noticed that the 

 side bearing the tooth and ridge is 

 always toward the outside of the 

 spiral (2 and 4 are supposed to lie 

 in the plane of the paper, while 1 is 

 below and 3 is above this plane). If 

 x-y is a plane surface to which the 

 spiral path is tangent, it will be ob- 

 served that the side bearing the 

 tooth is the only one to touch the 

 plane. 



Now, as we have already seen, many organisms 

 which habitually make use of this method of progres- 

 sion have a form which is adapted to it. In this way 

 is to be explained the asymmetry of the Infusoria. 



In the Rattulidce we have a group of animals, fun- 

 damentally bilateral, which are taking on the spiral, un- 

 symmetrical form as an adaptation to their method of 

 movement. 



If we examine in detail the behavior of one of the 

 Rattulidae, taking for example Diurella tigris Miiller 

 (Fig. E, 1), we find that it swims through the water in 

 a spiral, of such a course that its twisted body forms a 

 segment of the spiral path (Fig. I). The animal re- 

 volves to the right and swerves continuously towards 

 its dorsodextral side as it progresses. The result is a 

 path almost exactly that which would be formed if the 

 animal were moving on the inside of a hollow cylinder 

 and the dorsodextral spiral ridge ran in a groove on 

 the inner surface of the cylinder, which fitted it pre- 

 cisely 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 in the axis of flight. The result is, here as in the 

 rifle-ball, to make the axis of movement a straight line. 



It is apparent, therefore, that the general form of 

 the body is exactly adapted to the path which the body 

 follows through the water. And this general form is 

 produced by the twisting of the body from its original 

 bilateral symmetry into the condition already minutely 

 described. The reason why only the right half of the 

 striated area is, as a rule, developed into a ridge, which 

 slopes to the right, and why the ridge has an oblique 

 course are entirely evident from the method of move- 

 ment. The Rattulidae always revolve to the right as 

 they swim, so that the ridge sloping over to the right 

 is exactly fitted for aiding this movement; if both sides 

 of the area were elevated, the left one of the ridges so 



