LOCOMOTION IN GASTROPODS 161 
without sucking air through the perforation, thus demonstrating 
that its attachment in locomotion, as in rest, is due to adhesion 
and not to suction. In fact ina creeping Helix the foot not only 
does not suck but actually presses on the substrate. If, as the 
snail creeps, a bubble of air is intreduced under it by a capillary 
tube or other means, this air will usually escape at the edge of the 
foot in such a way as to show that it was under considerable pres- 
sure. The action of such bubbles demonstrates that the foot as 
a whole is firmly attached to the mucous substrate, in fact presses 
against it. Locomotion in Helix pomatia, then, has to overcome 
under ordinary circumstances only the adhesion of the foot and 
this is accomplished apparently by the pedal waves. In snails 
in which the attachment is due to suction as well as to adhesion, 
locomotion requires that both attractive forces shall have been 
overcome, but, as suction is muscular, it seems likely that this 
would be relaxed somewhat, as seems to be the case in Crepidula, 
before locomotion begins. 
How the pedal waves accomplish locomotion is still a disputed 
question. According to von Uexkill (09, p. 181), who has fol- 
lowed Jordan (01) and Biedermann (’05) in many particulars, 
each pedal wave is formed by the contraction of the longitudinal 
muscles of the foot and takes the form of a slight swelling on the 
underside of the organ. Such a wave, as von Uexkiill rightly re- 
marks, would effect nothing by way of locomotion unless some 
portion of the foot were fixed. Von Uexkiill (09, p. 187) believes 
that the foot is provided with some such mechanical device as the 
setae of the earthworm, which, resist backward movement while 
they allow forward motion and that, therefore, the region in front 
of each wave may be regarded as a fixed region. Hence the con- 
traction waves would always draw that portion of the foot where 
they temporarily were forward over the substrate toward the 
fixed point in front and as a result forward locomotion would be 
accomplished. 
Although this explanation is free from mechanical objections, 
it is doubtful whether it really applies to the case in hand. Von 
Uexkill has maintained in support of this view, that a snail can 
