USE OF THE FOOT IN SOME MOLLUSKS 361 



posteriorly, in a straight line, away from a source of light, and 

 its backward creeping may therefore be studied with ease. 



When Ischnochiton is creeping ventral side uppermost on a 

 slip of glass, it sometimes (and almost always if stimulated by 

 strong light) releases the anterior part of the foot, until finally 

 it remains attached only by means of about 2 sq. mm. of foot 

 surface. Hence it would appear that here, as in Chiton (Parker, 

 '14), the foot sucks locally. 



In moving anteriorly away from a strong light, the anterior 

 part of the body is sometimes completely freed from the sub- 

 stratum, then brought back to it, and then attached at the an- 

 terior end of the foot, initiating in this w^ay a 'giant wave,' 

 which, traveling the length of the foot, gives the appearance of a 

 'gallop,' probably analogous to that of Helix dupetithoutarsi and 

 of Aplysia (Parker, '11, '17), since it is retrograde, 'monotaxic,' and 

 involves the body musculature generally, not merely the foot. 

 Five or six waves of this character, one at a time, but sometimes 

 not quite coincident with the normal pedal wave (which may 

 therefore be seen independently), may be carried out in 

 succession. 



In these chitons the essential mechanism of progression is un- 

 doubtedly that outlined by Parker ('11) and illustrated on a gross 

 scale by the movements of Aplysia calif ornica (Parker, '17). I 

 have been able to verify this idea for a ditaxic foot, that of 

 Turbo. This mollusk also has a retrograde pedal wave which, 

 like that of Tectarius, is alternate ditaxic, anteriorly, but may 

 become opposite ditaxic after the first third of the foot has been 

 passed over. The foot is divided in the midline by a distinct 

 groove. When creeping on a vertical surface in air, it can be 

 clearly seen that the anterior region of one side of the foot is 

 first lifted 3 or 4 mm. from the surface, extended, and attached 

 at the anterior end; then the anterior portion of the opposite 

 side is moved similarly. This snail is an active creeper. The 

 foot measures 17 mm. x 25 mm., and the movements on its sur- 

 face can easily be followed. In creeping under water the pedal 

 waves are minute and frequently opposite ditaxic over the whole 



