144 G. H. PARKER 



they are attached narrows. If a fragment of shell or gravel is 

 taken hold of by forceps before it is naturally released,, it is 

 found to be held not by its whole surface but by a limited area 

 and with such force as to give the impression that it is held by 

 well localized suction. This view is fully confirmed by the 

 simple experiment of applying one's finger to the widened por- 

 tion of the foot of an inverted Aplysia, whereupon the finger is 

 taken hold of by the foot at several spots and the experimenter 

 has the sensation of strong but local suction at these spots. 

 Since the foot of Aplysia is practically free from mucous, it is 

 evident from these observations that its means of attachment is 

 by suction and that this suction is not by the foot as a whole, 

 as in Crepidula or Patella, but is produced locally. 



How local this suction is can be judged from the smallness of 

 the particles of shell or gravel that are found attached to the 

 foot. When an Aplysia is picked up from a shelly beach and 

 gently shaken in water, the largest fragments found attached to 

 its foot measure about 1 cm. in diameter. The smallest pieces 

 that are firmly held there have a diameter of approximately 

 2 mm., thus demonstrating that the suction areas must be of 

 very limited extent. Judging by the distribution of the bits 

 of shell and of gravel, suction may appear anywhere on the 

 full width of the foot and it ordinarily does appear over all that 

 part of the foot that widens in the course of locomotion. In 

 Aplysia, then, the widened part of the foot is the part that mo- 

 mentarily serves as a holdfast, and the foot is so organized that 

 its surface can temporarily and locally resolve itself into many 

 sucking organs capable of holding bodies whose diameters are 

 not above 2 mm. Doubtless this local suction is dependent 

 upon the activity of the perpendicular muscle strands in the 

 foot, as surmised by Jordan ('01, p. 197). 



In conclusion it may be stated that pedal locomotion in 

 Aplysia is due to monotaxic retrograde waves, which lift the foot 

 locally and temporarily from the substrate enabling it thus to 

 move forward with freedom while the rest of the foot for the 

 time being holds the snail in place by many small areas of local 

 suction. The observations on which these conclusions are based 



