USE OF THE FOOT IN SOME MOLLUSKS 365 



and posterior ends and the lateral margins remain firmly attached. 

 The body musculature then contracts, pulling the whole shell 

 forward into such a position that the aperture is immediately 

 over the foot. A 'step' of 3 cm. can thus be taken in about one 

 minute. Usually, when a freshly collected specimen is used, five 

 or six steps in a more or less continuous series occur in succes- 

 sion. The direction of progression is readily altered by the body's 

 being extended at an angle, previous to the attachment of the 

 foot. I did not obtain any evidence of backward steps. The ac- 

 tion of the foot as a sucker is clearly seen when the animal is at 

 rest, in which case the foot is not attached to the substratum at 

 all, but its middle third is distinctly arched, as it is in preserved 

 specimens. 



This type of locomotion is not exactly comparable with that 

 of a measuring worm, although in a sense these movements are 

 not dissimilar. The creeping of Xenophora under normal con- 

 ditions is probably rhythmic, but involves the body musculature 

 generally, and is in that sense non-pedal, although the suction of 

 the foot is essential to progression. Perhaps this type of loco- 

 motion is for the present better classed with the 'gallop' of Helix 

 (Carlson) and of Ischnochiton, although here again there are 

 distinctive features. 



SUMMARY 



This paper adds a further type of locomotion to those known 

 among gastropods (the 'looping' of Xenophora), provides a second 

 good example of arythmic pedal progression (Conus), and adds 

 three species of chitons (Ischnochiton, Acanthochites, Tonicia), to 

 the list of those known to move by means of retrograde pedal 

 waves. Ischnochiton moves posteriorly, preserving the retrograde 

 characacter of its pedal waves, with considerable freedom and for 

 appreciable distances. Ischnochiton also exhibits a 'gallop,' like 

 that of Helix (Carlson), which is independent of the pedal waves. 



Dyer Island, Bermuda 

 May 15, 1918 



