If a snail be placed on a piece of glass and made to crawl, it will be seen that a series of waves appear to pursue one another along the under surface of the foot, travelling from back to front in the direction in which the animal is moving. Simroth has shown that the sole of the foot is covered with a dense network of muscular fibres, those which run longitudinally being chiefly instrumental in producing the undulatory motion. By means of these muscles the sole is first elongated in front, and then shortened behind to an equal extent. Thus a snail _slides_, not on the ground, but on its own mucus, which it deposits mechanically, and which serves the purpose of lubricating the ground on which it travels. It has been calculated that an averaged sized snail of moderate pace progresses at the rate of about a mile in 16 days 14 hours.[102] =Sudden Appearance of Mollusca.=--It is very remarkable to notice how suddenly Pulmonata seem to appear in certain districts where they have not been noticed before. This sudden appearance is more common in the case of fresh-water than of land Mollusca, and there can be little doubt that, wherever a new pond happens to be formed, unless there is something in its situation or nature which is absolutely hostile to molluscan life, Mollusca are certain to be found in it sooner or later. “Some 23 years ago,” writes Mr. W. Nelson,[103] “I was in the habit of collecting shells in a small pond near to the Black Hills, Leeds. At that time the only molluscan forms found there were a dwarf form of _Sphaerium lacustre_, _Pisidium pusillum_, _Planorbis nautileus_, and _Limnaea peregra_. About 10 years ago I resumed my visits to the locality, and found, in addition to the species already enumerated, _Planorbis corneus_. These were the only species found there until this spring [1883], when, during one of my frequent visits, I was surprised to find _Physa fontinalis_ and _Planorbis vortex_ were added to the growing list of species. Later on _Pl. carinatus_, _Limnaea stagnalis_, and _Ancylus lacustris_ turned up; and during June, _Pl. contortus_ was found in this small but prolific pond.” _Limnaea glutinosa_ is prominent for these remarkable appearances and disappearances. In 1822 this species suddenly appeared in some small gravel pits at Bottisham, Cambs., in such numbers that they might have been scooped out by handfuls. After that year they did not appear numerous, and after three or four seasons they gradually disappeared.[104] _Physa_ (_Aplecta_) _hypnorum_ is noted in a similar way. In February 1852, for instance, after a wet month, the water stood in small puddles about 3 feet by 2 in a particular part of Bottisham Park which was sometimes a little swampy, though usually quite dry. One of these puddles was found to contain immense numbers of the _Aplecta_, which up to that time had not been noted as occurring in Cambridgeshire at all.[105] In a few days