142 G. H. PARKER 



end of the foot, as already described, and sweep steadily over 

 this organ to its posterior end, the snail meanwhile making a 

 relatively gigantic forward stride, as Carlson ('05) has described 

 for Helix dupetithouarsi. As one wave disappears at the poste- 

 rior end, another appears anteriorly. At the initiation of loco- 

 motion the head of the snail together with the subjacent portion 

 of the foot is lifted well off the substrate (fig. 1, A) and projected 

 forward narrowing as it extends till it has been advanced a dis- 

 tance equal to a fourth, a third, or even a half the length of the 

 animal. Then the anterior edge of the foot is brought down on 

 the substrate and attached while the foot posterior to this part 

 forms an arch reaching back to the hind end of the snail (B). 

 This end, though still attached to the substrate, is just about 

 to be freed, being in fact the vanishing traces of an area of at- 

 tachment such as is now established at the front end. The 

 anterior portion of the arch of the foot now crowds forward and 

 attaches itself behind the attached anterior edge and thus the 

 arch itself seems to move backward till the posterior portion 

 of the animal is released from the substrate and is crowded and 

 carried forward a distance equal to that over which the head 

 was advanced. With the disappearance of this wave at the 

 hind end of the snail, a new wave starts at the anterior end and 

 so on in regular succession. 



Where on the foot of the creeping gastropod locomotion is 

 actually accomplished, has been a matter of uncertainty. In 

 Aplysia, however, there can be not the least doubt that the an- 

 terior part of the arch of the foot is the portion that moves 

 forward. The advance of this region and the consequent crowd- 

 ing together of its parts is easily observable, and these changes 

 occur in no other area of the foot. All this is easily seen in 

 Apylsia, not only because of the large size of its pedal wave 

 but in consequence of the relatively great height to which the 

 arched portion of its foot is momentarily lifted. In many 

 snails it is very difficult to demonstrate that the moving portion 

 of the foot is lifted at all from the substrate; in fact Biedermann 

 ('05, p. 10), who studied Halix pomatia with much care, was 

 erroneously led to believe that this part was actually pressed on 



