The ordinary mode of locomotion of the testaceous sea-snails is by creeping along on their foot: those that have a very heavy house to carry, such as the Cassis or the Pteroceras, generally move along very slowly, while others, such as the Olivæ, that are possessed of a comparatively strong and broad foot, have rapid and lively movements, and quickly raise themselves again when they have been overturned. The Strombidæ and Rostellariæ place their powerful and elastic foot under the shell in a bent position, when suddenly by a muscular effort they straighten that organ and roll and leap over and over. The structure of the foot of the _Tornatella fasciata_, an inhabitant of our coast, is most remarkable: beaten incessantly by the waves, in the cavities of rocks which it frequents, nearly on a level with the surface of the sea, to the violence of which it is always exposed, it has need of additional powers for retaining its hold; its foot is therefore divided into two adhering portions, placed at each extremity, and separated by a wide interval; when it crawls, it fixes the posterior disc and advances the other, which it attaches firmly to the place of progression, and this being effected, the hinder sucker is detached and drawn forwards, locomotion being accomplished by the alternate adhesion of these two prehensile discs. In Cyclostoma the foot is likewise furnished with two longitudinal adhering lobes, which are advanced alternately. But the foot of the marine snails is not merely an instrument of progression on a solid surface, for in many species it is convertible at the will of the animal into a boat, by means of which the creature can suspend itself in an inverted position at the surface of the water, where by the aid of its mantle and tentacles it can row itself from place to place. [Illustration: Pteroceras scorpio.] [Illustration: Oliva hispidula.] [Illustration: Strombus pes pelicani.] [Illustration: Ianthina communis.] The Ianthinæ, or purple Sea-Snails, carry under their foot a vesicular organ like a congeries of foam-bubbles, that prevents creeping, but serves as a buoy to support them at the surface of the water. [Illustration: Murex haustellum.] When the sea is quiet, these little creatures, Like little wanton boys that swim on bladders, appear in vast shoals on the surface, but as soon as the wind ruffles the ocean, or an enemy approaches, they at once empty their air-cells, contract their float, and sink to the bottom, pouring out at the same time a darkened fluid like that of the Aplysia or the Murex, which no doubt serves them as a defence against their foes, and, according to Lesson, furnished the celebrated purple of the ancients. The Ianthinæ inhabit the Mediterranean and the warmer regions of the Atlantic, but especially towards the close of summer they are frequently drifted by the Gulf Stream to the west coast of Ireland. [Illustration: Magilus antiquus.] [Illustration: Worm-Shell.]