378 THE AMERICAN NATURALIST. 
Revolution on the long axis is also to be observed in many 
rhabdoceels. It is indeed one of the commonest features in 
the locomotion of small fresh-water organisms, and doubtless 
occurs in salt-water forms in the same way. 
On the whole, then, it is clear that revolution on the long 
axis, with the resulting spiral path, is of high biological 
significance. Only through this device are many organisms 
enabled to follow a course which is practically a straight one; 
without such revolution many creatures merely describe small 
circles, making no progress whatever. By means of this revo- 
lution on the long axis, any organism, no matter how misshapen 
and irregular, may follow a course which is, in effect, equivalent 
to a straight line. The simple device of revolving in the axis 
of progression is surprisingly effective, in that it compensates 
with absolute precision for any tendency or combination of 
tendencies to deviate from a straight course in any direction 
whatsoever. 
ANN ARBOR, MICH, January 5, 1901. 
