LOCOMOTION IN GASTROPODS 163 
the same type of movement as has been described for the specks 
in the foot of Helix. In Limax the waves, however, are light in 
color, instead of being dark as in Helix, and their surfaces, .as 
seen in the air, are marked with fine wrinkles transverse to the 
longitudinal axis of the animal. These wrinkles show that the 
waves are regions of longidudinal contraction, as has been main- 
tained by most recent writers on this subject. 
The chief error in most previous accounts of the locomotion of 
the gastropod foot is found in the physical configuration ascribed 
to the underside of this organ. Biedermann (05, pp. 10, 17) 
states that the waves are convexities on the surface of the foot 
and that they press more firmly against the substrate than does 
the rest of the foot. This view was adopted by von Uexkill 
(09, p. 187) in his discussion of gastropod locomotion. In Helix 
pomatia it is by no means easy to determine whether the waves 
are convexities or not, for the reason that they are at most only 
very slightly different in level from the general surface of the 
foot. On inspecting by reflected light the free ventral surface of 
a part of a Helix foot over which waves were running, I was un- 
able to tell with certainty whether the surfaces of the waves were 
convex, concave, or flat. If, however, the creeping foot be closely 
studied through glass, evidence of a conclusive kind can be found. 
If, under these circumstances, a very minute air bubble entangled 
in the mucus under the snail is watched, it will be seen to change 
its form and position slightly as each wave passes over it. As 
the wave approaches it, it will elongate slightly on its face 
next the wave and at times move a little towards the wave, and 
as the wave leaves it, it will elongate slightly in the opposite 
direction and at times follow slightly the retreating wave. The 
motions of the bubble are exactly those that should be expected 
provided the wave exerted a slight suction in its passage and the 
reverse of what would occur supposing the wave pressed upon the 
bubble. The evidence, though slight, is clear and I, therefore, 
believe that each wave on the underside of the foot of Helix 
pomatia is a slight concavity. 
Although the configuration of the surface of the wave in Helix 
pomatia could be determined only indirectly, in Limax maximus 
