The limpet adheres to rocks by a great disk-like mass of muscle, which is called "the foot." It is really the whole ventral surface, and it can loosen its hold, and, by curious ripples of contraction, cause the animal to creep or glide over the rock. At low tide the limpet is exposed to the air, and remains motionless, but when the tide is up it makes a small excursion in search of food, never going more than a foot or two from the spot which it has chosen, and returning to it, so that in the course of time it actually wears away a sort of cup or depression at this spot--if the rock is not of exceptional hardness. The word "foot" is applied to the ventral disk-like surface of the limpet, because in many univalves this region becomes drawn out, and is connected by a comparatively narrow and nipped-in stalk or pillar with the rest of the animal. This occurs in the univalves which have large spiral shells, into which the whole of the soft animal can be deeply withdrawn, which is not the case with the limpet. You may find on the shore at Torquay a sea-snail (Natica), in which the animal is quite invisible, drawn far up into the shell. Place this in sea-water and watch it. Soft semi-transparent lobes begin to issue from the mouth of the shell, part of the soft distensible foot appears swelling out and growing bigger and bigger, and soft folds spread out from the mouth of the shell, and gently creep over it, and completely envelop it; the foot begins to grip the bottom of the vessel, and the animal "crawls." At last, swelling out from the other folds of soft but tense "molluscan" substance, the head and its tentacles emerge. Touch the animal and it shrinks rapidly, disappearing into the shell.