LOCOMOTION IN GASTROPODS 167 
point on the foot is lifted, moved forward, and set down again 
and thus the foot, and with it the animal as a whole moves for- 
ward. From this theoretic consideration, it is evident that the 
theory of pedal-wave action advanced in the preceding paragraphs 
is entirely consistent with such an arrangement of muscles as has 
long been known to occur in the gastropod foot. 
Vlés (07) has called attention to the fact that the majority of 
theories as to the locomotor action of the gastropod foot apply 
only to the direct type of movement and do not take into account 
the retrograde type. The theory put forward in this paper is 
believed to apply equally well to both types. Among retrograde 
gastropods, Chiton tuberculatus is an excellent example. This 
molluse uses its foot asa sucker, but nevertheless can creep with 
considerable rapidity. It exhibits, as a rule, not more than two 
waves on the foot at a time; these course posteriorly at the rate 
of about a centimeter in five seconds. In a Chiton creeping over 
a glass plate, the wave when viewed from the side can be seen to be 
an area lifted well off the substrate. This feature is much more 
conspicuous in Chiton than in any other mollusc that I have ex- 
amined. As in the pulmonates, the surface of the Chiton foot in 
direct. contact with the substrate is motionless; that in the wave 
area moves forward. At any moment about one quarter of the 
Chiton foot is moving forward while the animal supports itself on 
the remaining three quarters. 
In Dolabrifera the foot is pear-shaped in outline with the 
rounded end posterior. Itisabout8mm.inlength. In creeping, 
one to two waves can be seen on its surface at once; each wave 
sweeps the length of the foot in aboutseven seconds. Asin Chiton, 
the waves can be clearly seen to be areas in which the foot is lifted 
completely from the substrate to which the rest of the foot is 
firmly applied. The pedal surface is mottled and in the wave 
area it can be seen to be moving forward, whereas on the rest of 
the foot it is motionless. The total wave area is about one-half 
the total area of the foot. 
The conditions in Chiton and in Dolabrifera are essentially 
similar to those in the pulmonates, except that the pedal waves 
progress posteriorly instead of anteriorly, z.e., the dorso-ventral 
