LOCOMOTION IN GASTROPODS 165 
Tectarius, as already noted, practically fulfills these conditions 
except that its waves are retrograde. This general theory of 
the mechanics of gastropod locomotion is an elaboration of the 
views already set forth by Jordan (01). 
It is not my purpose in this paper to enter into an account of 
the musculature by which the movements already described are 
carried out, for I have made no observations on this part of the 
subject. It is, however, pertinent to show that the elements of 
motion implied in the preceding description are not inconsistent 
with the general structure of the snail’s foot. The work of Jor- 
dan (01), Biedermann (’05), and others shows conclusively, I 
believe, that the musculature of the snail’s foot works against 
the elastic-walled, fluid-filled cavities of the animal’s interior 
and that these cavities are often temporarily closed from one 
another. It is these spaces which, acting collectively as a vacuo- 
lated, erectile tissue, give rise to such rigidity as is possessed by 
the expanded foot of the snail. In this tissue two sets of mus- 
cles, longitudinal and dorso-ventral, have been identified. The 
dorso-ventral muscles lift the foot locally from the substrate. 
They are imbedded in the vacuolated tissue already mentioned 
and when they contract, their dorsal ends, being more firmly 
set than their ventral ones, serve as relatively fixed points and 
the ventral ones, therefore, move. The mechanical support that 
these muscles receive comes primarily from the tissue adjacent 
to their dorsal ends which in turn gets its support from other 
tissues reaching to the parts of the foot fixed on the substrate in 
front and behind the region of elevation. The action of the ven- 
tral end lifts the foot locally and overcomes adhesion in the given 
region. When the muscle relaxes, the portion of the foot that was 
elevated is returned to its former level chiefly by the elastic action 
of the vacuolated tissue and the muscle recovers its original length 
and position. This action of the dorso-ventral muscles takes 
place in sequence from behind forward and thus a concave wave 
runs on the surface of the foot from tail to head. 
The second element in the pedal wave is the forward movement 
of that portion of the foot which is temporarily lifted from the 
substrate. This must be accomplished by the contraction of the 
