CILIARY AND MUSCULAR LOCOMOTION. 137 



tinuous though slight muscular contractions. It was conceivable, 

 therefore, that it played some important part in effecting the loco- 

 motion of the animal which had been overlooked. In order to test 

 this possibility the anterior portion of a snail's foot was removed 

 so that the locomotor function became restricted to the mid and 

 hind regions. Less than three days after the operation the snail 

 was moving about the aquarium so much like other individuals that 

 it could not be distinguished from them without examining its foot. 

 \Yhen moving rapidly, ten records of its speed were taken which 

 showed an average locomotor rate of 5 mm. in 3 seconds. Five 

 records obtained at other times when it was most active averaged 

 2.6 seconds for the same distance. Another, normal animal, whose 

 speed of locomotion was noted before the preceding tests were 

 made, was moving at the rate of 5 mm. in 3.2 seconds (average of 

 ten trials) . After completing this experiment more of the anterior 

 region of the foot was cut away so that there was nothing left 

 which came in contact with the substrate which could possibly be 

 regarded as a portion of the propodium. Again locomotion was 

 not interfered with, and the snail glided both slowly and rapidly 

 over the bottom of the aquarium. It is clear, therefore, that the 

 propodium is not a part of the pedal mechanism which is essential 

 for locomotion. 



MUSCULAR LOCOMOTION. 



In sharp contrast to the ciliary type of locomotion in Poliniccs 

 is one involving the formation of rhythmic pedal waves. This is 

 a muscular form of progression which begins by the posterior end 

 of the foot being drawn forward initiating a dark transverse band 

 or wave, which travels over the ventral surface of the foot until it 

 reaches the anterior portion when the propodium is thrust ahead. 

 At about the time the propodium is fully extended the hind end of 

 the foot is again contracted and a second wave is produced. This 

 muscular action causes a slight humping of the body, so that the 

 shell rises after the posterior contraction and falls again as the 

 wave passes forward. If the animal is moving very rapidly the 

 anterior end of the foot may be lifted well above the substrate 

 when the wave reaches it, extended and brought down again with 

 considerable force. Although the wave may not be apparent along 



