318 ASYMMETRY IN CERTAIN LOWER ORGANISMS, 



the other hand, the two sides, right and left, have similar relations to the environ- 

 ment, there being no influence which acts upon one differently from the way it 

 affects the other. Hence the two sides are alike. 



Now, besides the radially symmetrical and bilaterally symmetrical forms, we 

 may distinguish another type of structure, having an equally definite relation to 

 the method of life, which has hitherto not been recognized, at least not as having 

 a definite relation to a widespread method of movement and life. This is what may 

 in general be characterized as a spiral type of structure, or at least as a one-sided, 

 unsymmetrical, type. This type of structure is found in organisms which move 

 in such a way that no two sides have the same relation to the environment. 



Movement of this sort is found in those organisms which swim in a spiral course. 

 Such, for example, are the free-swimming Ciliata and Flagellata; such also are most 

 of the Rotifera and many other small aquatic organisms. In order to appreciate 

 why unsymmetrical structure should be characteristically associated with spiral 

 movement, it will be necessary to examine the nature of such movement (see Jen- 

 nings, :01). 



When a minute organism starts to swim through the open water in a given direc- 

 tion there is an indefinite number of possibilities that it may go astray. To follow 

 a straight line is difficult in any case, and in the free water it is possible to swerve 

 from the straight line not only to the right or to the left, but also up or down or in 

 any intermediate direction. Most of these lower organisms do not have image-forming 

 eyes, so that we cannot suppose them to be guided at every stage by sight. In the 

 open water they are therefore in much the same situation as the wanderer in a bliz- 

 zard on a trackless prairie, with the added possibility of wandering from the course 

 as well up or down as to the right or left. 



Under these conditions it would require a most accurately adjusted structure 

 to avoid swimming in a circle. Any small imperfection or difference in the form 

 or structure of one side of the animal would cause a swerving toward or away from 

 that side, and the result would be movement in a circle, so that no progress would 

 be made. As a matter of fact, as we shall see, most of these organisms are more or 

 less one-sided. 



So, if there were no special device to avoid this difficulty, most of these organ- 

 isms would swim in circles, making no progress. But there is a special device which 

 does avoid the difficulty. The organisms revolve on their long axes as they swim. 

 As a necessary mechanical result, the curved course becomes a spiral one (Fig. A). 

 The animal continually swerves from the straight line toward one side, but, owing 

 to the revolution in the axis of progression, this side takes continually a new posi- 



