PEDAL LOCOMOTION OF APLYSIA CALIFORNICA 143 



the substrate. But in Aplysia calif ornica the arch of the foot is 

 sometimes lifted as much as 1.5 cm. above the surface over 

 which the snail is moving thus enabling an observer when in a 

 favorable position to see completely under the animal. Hence 

 it is perfectly easy to observe that the region of the foot that 

 moves forward is the elevated region and that the most active 

 part of this region is the anterior part. 



The second question of importance in the locomotion of gas- 

 tropods is where and how the foot is attached to the substrate 

 during locomotion. Two methods of attachment have long been 

 known; either the snail holds to the substrate by suction, or a 

 bed of mucus is laid which on the one hand adheres to the sub- 

 strate and on the other to the animal's foot. In Aplysia, as in 

 all other gastropods, the portion of the foot that serves as a 

 holdfast is of course within the limits of the area temporarily 

 applied to the substrate. That part of the Aplysia foot 

 temporarily in contact with the ground is formed, as already 

 indicated, by the crowding together of the anterior region of the 

 arch. In consequence of this crowding the part of the foot ap- 

 plied to the substrate is always broader than the portion that 

 is momentarily free from contact. At the moment the middle 

 of the foot is attached and the two ends are free, the foot has the 

 form shown at A' ', figure 1, broad in the middle and narrow an- 

 teriorly and posteriorly. On the other hand, at the moment the 

 middle of the foot is free and the two ends are attached it has 

 the form shown at B', narrow in the middle and broad at the 

 two ends. Thus in the progress of the waves from the head to 

 the tail of Aplysia the area including the region of attachment is 

 marked by considerable breadth, whereas that portion of the foot 

 that is free is always narrow. 



The means of attachment in the widened portion of the foot 

 can be demonstrated very clearly on an Aplysia that is creeping 

 over a shelly or gravelly beach. If such an animal is quickly 

 inverted, the momentarily narrow part of the foot will be found 

 to be quite clean, whereas the broadened part will be seen to be 

 covered with a great number of fragments of shell and gravel, 

 all of which drop off as soon as the region of the foot to which 



