168 G. H. PARKER 
muscles contract in sequence from the anterior to the posterior 
end instead of the reverse and the longitudinal muscles follow 
the same sequence; otherwise they act as they do in the direct 
type. It is evident from this brief discussion of the nature of the 
waves in the retrograde type that the theory developed in connec- 
tion with the direct type applies perfectly to this second type. 
It remains still to point out that what I have called the arhyth- 
mic form of pedal locomotion, a form well exemplified in Ilyan- 
assa, may be explained on the same general basis as that which 
has just been given for the two types of arhyhmic locomotion. If 
the foot of such a snail as Ilyanassa be thought of as composed of 
a multitude of small areas, each one of which can be lifted from 
the substrate, moved forward, and set down again separately, 
and that this action takes place irregularly and without reference 
to any sequence, it can easily be seen how the animal could move 
forward but without the formation of pedal waves. It is my be- 
lief that this is the condition in the foot of the arhythmic gastro- 
pods, but because of the small size of Ilyanassa, I have not been 
able to subject this opinion to experimental test. 
Before closing this paper, I wish to add a word concerning the 
very remarkable method of locomotion observed by Carlson (’05) 
in Helix dupetithouarsi. The movement carried out by this 
snail is appropriately described as a gallop, both from its rate and 
configuration. The snail on strong provocation lifts the head and 
projects it forward, and eventually brings it to the ground, thus 
initiating a giant wave which proceeds backward over the length 
of the body. Several such waves may be present at once. Carl- 
son suggests that this movement is only an exaggerated form 
of the ordinary locomotion, but I am inclined to agree with Jordan 
(05, p. 104) that this is probably an entirely different type of 
locomotion and I suspect that this snail also possesses the typical 
pedal wave. In fact it seems to me likely that the gallop was, 
so to speak, superimposed on the pedal wave system and, had the 
snail when in gallop been examined from below, the pedal waves 
would have been seen in operation in conjunction with the body 
waves. I am the more inclined to the view that the gallop is an 
independent form of locomotion as compared with the pedal 
