162 G. H. PARKER 
be slipped over a glass plate more easily forward than backward, 
just as an earthworm can be drawn over an appropriate surface 
more easily headward than tailward. I must confess that I have 
not been able to convince myself that there is any difference 
in this respect in Helix pomatia or Limax maximus; both 
seem to slip over the glass forward and backward with equal 
ease. 
Moreover, the view advanced by von Uexkiill is based upon 
what I believe to be a somewhat erroneous conception of the pedal 
wave. Biedermann (’05, p. 11) pointed out that the foot of Helix 
pomatia has great advantages over that of many other gastro- 
pods for studies of this kind because of the numerous small specks 
contained in its outer layer. These specks can be discerned clearly 
by means of a hand lens and they give a true picture of the move- 
ments of the foot. As watched through a plate of glass over which 
the animal is creeping, they can be seen, as Biedermann has de- 
seribed, to move momentarily forward, then come to rest, and then 
again to move forward. ‘This is best demonstrated on a sheet of 
glass on which there are numerous scratches. Such scratches 
serve as landmarks and by them it can be seen that the minute 
specks in the foot do remain essentially fixed in position and then 
momentarily move forward to assume again for a brief period a 
position of rest. When this motion is examined in relation to the 
foot as a whole, it is evident that the forward motion takes place 
in the dark waves and that quiescence is characteristic of the 
intermediate lighter portions of the foot. Each wave, then, is a 
pulse of forward motion and the rest of the foot is momentarily 
quiescent. The area covered by the waves is probably a fourth - 
or a fifth of the total area of the foot. At any moment, therefore, 
about three-fourths to four-fifths of the surface of the foot is 
stationary and about one-fourth to one-fifth is moving forward. 
In other words the snail stands on the greater part of its foot while 
it moves forward with a much lesser part. 
Essentially the same conditions as have been described for Helix 
pomatia can be demonstrated in Limax maximus. I particles 
of carmine be driven into the substance of the median, active 
band of the foot of this slug, they can be seen to exhibit exactly 
