However different the form of a shell may be, its use is invariably the same, affording the soft-bodied animal a shield or retreat against external injuries. In this respect it is not uninteresting to remark that those species which inhabit the littoral zone, and are most exposed to the violence of the waves, have a stronger shell than those which live in greater depths, and that the fresh-water molluscs have generally a much more delicate and fragile coat than those which live in the ocean. The greater the necessity of protection the better has Nature provided for the want. Thus most of the gasteropods, besides possessing a stone-hard dwelling, are also furnished at the extremity of the foot with an operculum, or calcareous lid, which fits exactly upon the opening of their house, and closes it like a fortress against the outer world. But no animal exists that is safe against every attack, for the large birds sometimes carry the ponderous sea-snails, whose entrance they cannot force with their beaks, high up into the air, and let them fall upon the rocks, where they are dashed to pieces.
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.
Pteroceras scorpio.
Oliva hispidula.
Strombus pes pelicani.
Ianthina communis.