140 G. H. PARKER 



end of that organ and sweep over it posteriorly 1 ; they involve 

 not only the pedal muscles proper, but more or less of the mus- 

 culature of the adjacent body wall. In consequence of the 

 direction and extent of these waves they may be designated as 

 retrograde monotaxis, to use terms introduced by Vies ('07). 



In Aplysia californica as a rule there is present on the foot 

 only one wave at a time (fig. 1, B), and as this wave dies out at 

 the posterior end, a new one starts at the anterior end (A). 

 Occasionally a wave on approaching the posterior limit of the 

 foot decreases its rate of progress and before it has disappeared 

 at the hind end, a second wave makes its appearance at the 

 anterior end thus giving rise to a condition in which two whole 

 waves may be represented on the foot at the sajme time. This 

 condition, however, is distinctly exceptional, for as a rule not 

 more than one wave can be seen on the foot at any one moment. 



In an Aplysia whose length of body was about 23 cm., 19 waves 

 passed over the foot in 2 minutes and the animal progressed in 

 that time 124 cm.; or, expressed in averages, a wave appeared 

 every 6.3 seconds and the snail progressed with each wave 6.5 

 cm. In another Aplysia 26 waves passed over the foot in 225 

 seconds, during which time the snail went forward 135 cm.; or, 

 again expressed in averages, a wave occurred every 8.7 seconds 

 and the animal advanced 5.2 cm. with each wave. It is thus 

 clear that in the ordinary locomotion of Aplysia californica wave 

 follows wave about every 6 to 8 seconds and with each wave the 

 snail progresses 5 to 6 cm. In some instances, however, progress 

 was much more marked than these figures indicate; thus an 

 Aplysia with a length of body of 23 cm. was seen to advance 

 8 to 10 cm. for each wave and on one occasion as much as 13 

 cm. or a little more than half the length of its body. When one 

 compares waves of these dimensions and effectiveness with the 

 small ripple-like movements seen on the foot surfaces of most 

 gastropods, the advantages offered by Aplysia for the study of 

 pedal locomotion are evident. 



1 My observations on the direction taken by the pedal waves in Aplysia cali- 

 fornica agree with those of Jordan ('01, p. 199) on A. limacina. Vies' ('07, p. 

 277) general statement that in Aplysia the pedal waves appear to run from pos- 

 terior to anterior is very probably inaccurate. 



