AND ITS BIOLOGICAL SIGNIFICANCE. 



319 



tion. The divergence from the straight line is therefore 

 compensated, and the path becomes a spiral. A spiral has 

 of course a straight axis, so that the organism can make 

 progress in this way as well as by swimming without any 

 divergence whatever. 



Thus the spiral course may be considered as a very 

 simple device to enable an organism to make progress in 

 a given direction through the free water without fulfilling 

 the difficult condition of making all sides identically alike, 

 or of making the differences exactly balance each other. 



II. INFUSORIA. 



Fig. A. Fig. B. 



Fig. A. — Spiral Path of Paramecium; the 

 aboral side always toward the outside of 

 the spiral. (1 and 3 are supposed to lie 

 in the plane of the paper, 2 above, and 

 4 below this plane.) 



Fig. B. — Paramecium, after Butschli, show- 

 ing spiral structure. 



As will appear from the foregoing discussion and from 

 Figure A, the organism swimming in a spiral course keeps 

 its body in a definite position with relation to the axis of 

 the spiral. The same side always faces the outside of the 

 spiral; the opposite side always faces the axis. Thus in 

 Paramecium (Fig. A) the oral groove always faces the axis 

 of the spiral, while the aboral side always looks to the 

 outside. Paramecium thus swerves continually 

 toward its own aboral side. 



Now, many of the organisms that swim in 

 such a spiral path have the body so formed as 

 to adapt it to this motion. This is the case, for 

 example, in Paramecium (Fig. B). Here the 

 body has really a partially spiral form. The 

 oral groove, as is well known, passes in a curve 

 from the middle of the body obliquely toward 

 the anterior end and to the left. The animal 

 follows a spiral path, of such a nature that the 

 oral groove forms a segment of the spiral, the 

 groove always facing the axis of the spiral. 

 The body is thus unsymmetrical; it cannot be 

 divided by any plane into similar halves. 



An unsymmetrical form is characteristic for 



