LOCOMOTION IN GASTROPODS 159 
fully than the foot alone can adhere to the glass. That this 
attachment is due chiefly to the adhesive properties of the mucus 
and not to the sucking action of the foot, is seen from the fact that 
the attachment can be completely accomplished over a minute 
hole in a plate of glass. When a snail in such a position is seized 
and drawn off, air is sucked in through the hole in the glass as the 
middle of the foot rises, showing that under these extreme cir- 
cumstances, the foot does act as a sucker, but in the ordinary 
resting state of the snail no such suction is exerted. All snails 
with which I am acquainted deposit more or less mucus and 
though this is sometimes so small in amount that it can be demon- 
strated only by means of powdered carmine, it serves, I believe, 
in so far as it is present, as a means of attachment. This produc- 
tion of mucus is highly developed in the pulmonates. Its rela- 
tion to creeping on the surface-film of water, as exhibited by many 
fresh-water snails, has long been recognized. i 
In some snails the foot serves as an organ of attachment chiefly 
through its power of suction. The general surface of the foot is 
applied closely to the substrate after which the central portion 
is lifted thus converting the foot into a sucker. This kind of 
attachment is well exemplified in Patella, Crepidula, etc. Crep- 
idula fornicata can be made to creep over a surface of glass and 
can move with ease and security over a minute hole in the glass. 
If, however, the snail is disturbed by being touched several times 
when its foot is over the hole, it will actually dislodge itself by 
endeavoring to suck firmly to the glass, for in so doing it will 
fill to repletion the forming concavity on the underside of its foot 
by sucking water through the underlying hole. When one con- 
trasts the difficulty with which Crepidula is dislodged from its 
natural surface of attachment, particularly after it has been in- 
duced to exert full suction, with the ease with which it can be 
made to dislodge itself when over a small hole, the magnitude of 
its power of suction becomes apparent. The action of the foot 
of Aplysia as a suction apparatus has already been demonstrated . 
by Jordan (01). These two methods -of attachment, suction, 
and adhesion through mucus are the chief means by which snails 
hold to the surfaces on which they creep. 
