Page:Natural History, Mollusca.djvu/99

 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 traversing 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 Eolis 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