LOCOMOTION IN GASTROPODS Mea iF 
so extensive that never more than two waves can be seen on one 
side of the foot at once. The foot, therefore, moves forward in 
alternate steps, first on the right side and then on the left, the 
motion resembling that of a person in a sack walk. In Nerita 
the wave begins anteriorly as a single wave whereupon it breaks 
and passes down the right and left sides of the foot to unite as 
one wave again at the posterior margin. These two conditions 
of alternate waves, asin Tectarius, and opposite waves, as in Nerita. 
will probably be found exemplified in other ditaxic gastropods. 
In certain small species of Littorina with direct movements, Vlés 
has described four parallel sets of waves, fulfilling the require- 
ments of a tetrataxicsubtype. This occurs, according to Vlés, 
only in connection with the retrograde type of movement. I 
have seen no example of it. 
Among those snails that I have examined, one species, Ilyan- 
assa obsoleta (Say), seems to find no place in Vlés’ classification. 
This snail is a vigorous, active creeper. Its foot covers a large 
area compared with the size of its body. Anteriorly the foot is 
truncated and auriculate; posteriorly it is bluntly rounded. Its 
ventral surface is whitish, flecked over with irregular grayish 
splotches. In resting, the snail uses chiefly the posterior part of 
the foot, the anterior part being sometimes more or less with- 
drawn into the shell. In locomotion the anterior part seems to 
be the more active. Notwithstanding the fact that this snail 
is very easily observed in active creeping and that its foot is 
marked in a most favorable way for exhibiting wave-like move- 
ments, I have never been able to discover any evidence of such 
movements. When in locomotion, the whole foot seems to glide 
at a uniform rate over the surface of attachment such as that of a 
glass plate. Only along the anterior edge and over a small por- 
tion of the anterior ventral surface of the foot, can slight varia- 
tions in the rate of movement be discovered and these variations 
are so local and scattered that they can in no sense be regarded 
as forming a wave. The movement of the foot of Ilyanassa has 
a most striking resemblance to that of the foot of a planarian in 
which cilia may be the chief motor organs, but on testing the foot 
