164 G. H. PARKER 
it can be seen with distinctness. If the anterior part of the 
foot of this slug be applied to a glass surface, the pedal waves 
appear quickly over the whole foot. On inspecting the portion 
of the foot not yet in contact with the glass, the waves can be 
identified as dark bands alternating with light areas. On examin- 
ing from the side the portion of the foot not yet in contact with 
the glass, it can be clearly seen that the waves are concavities 
in the foot as compared with the areas between the waves. I 
am, therefore, entirely convinced that, contrary to the opinion 
expressed by Biedermann and others, the pedal waves of the gas- 
tropods are concavities and not convexities on the foot. In these 
concavities, which are probably filled with the more fluid portion 
of the mucus, the foot moves forward, the rest of this organ being 
temporarily at a standstill. 
The mechanical advantage of this arrangement must be obvious. 
The snail is attached to the substrate chiefly by adhesion to the 
denser mucus. This attractive force is overcome by drawing 
certain parts of the foot, the region of the waves, away from the 
substrate. These parts are then in a position to move with re- 
duced resistance and are momentarily shifted forward while the 
snail supports itself on the rest of its foot. As this release from 
adhesion is propagated as a wave over the whole of the foot, this 
whole organ, together with the rest of the snail, is eventually 
moved forward. At first thought it might seem that such a 
wave movement could not produce so uniform a motion as snails 
show, but it must be remembered that the uniformity of this 
movement is seen only in parts of the animal some distance from 
the foot. On the foot itself the operation is alternate movement 
and rest, which becomes more and more continuous motion as 
points onthe body moreand more distant from the foot arereached. 
The locomotion is in many fundamental respects like that of the 
human being. In our locomotion each foot is alternately at rest 
and in motion and yet distant parts of our body, like the head, 
show a motion which in comparison with that of our feet is almost 
continuously uniform. In fact, a ditaxic gastropod with alter- 
nate, direct, single waves on the foot would almost exactly re- 
produce the method of locomotion found in the human being. 
