POND-SNAILS. S7 
and generally crossed by transverse raised lines, 
giving it an angular appearance ; this whorl is large, 
and often greatly swollen. The pillar fold is 
strongly marked, almost forming an umbilicus. 
The animal is of a yellowish hue, paler beneath. 
In stagnant and slow moving waters this species 
is not uncommon; and, on a warm summer’s day, 
numbers of this and other species may be seen tra- 
versing the mud, climbing the aquatic plants, or 
mounting to the top, and floating on the unruffled 
surface in a reverse position. ‘The principle by 
which an animal, heavier than water, is thus enabled 
to float, is not very easy of explanation ; a parallel 
to it, however, is afforded by the familiar experiment 
of carefully laying a needle on still water, where 
it will float as long as it remains unshaken. The 
swimming body must, of course, be considered as 
in contact with the incumbent air, the cohesive 
power of which to the body, and among its own 
particles, is probably sufficient to overcome the 
force of gravity. But the Pond-snail not only 
floats, but swims at the surface, traversing its pool 
with a smooth, gliding motion, in an undulating 
line. M. de Quatrefages is of opinion that the 
progression of Mollusks, in this reversed position 
on the surface of the water, cannot be made by 
any muscular action of the foot; and he ascribes 
the motion to the action of the vibratile cilia, which 
cover the entire body as well as the sole of the foot.: 
Dr. Johnston,* however, sets in opposition to this 
opinion the fact, that an Hols crossing a basin can 
at once stop and remain there for any time, though, 
during all this period of rest, the cilia are in as 
. * Introd. to Conchol. 130. 
