934 DIVISION Il. MOLLUSCOUS ANIMALS.—CLASS HI. GASTEROPODS. 
necks. The giant Nerite commands any price; and many of the volutes, 
cones, mitres, and harps are purchased at a price exceeding several times 
their weight in gold.” 
However different the form of the shell may be, its use is invariably the 
same, affording the soft-bodied animal a shield, or retreat from injuries. 
In this respect it is not uninteresting to remark, that those species which 
inhabit the coasts, and are more exposed to the rolling of the waves, have 
a thicker and stronger shell than those which live in greater depths, and 
that the fresh water mollusks 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 larger sea-snails, besides possessing a stone-hard dwelling, are also 
furnished at the extremity of the foot with an operculum or calcareous lid, 
which fits like a door 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 sea-birds sometimes carry the ponderous 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 sea-snails is by ereeping along 
on their foot; those that have a very heavy house to carry, such as the 
Cassis, or the Pteroceras, move along very slowly, while others, such as 
the Olive, that are possessed of a comparatively strong foot, have rapid 
and lively movements, quickly raise themselves again when they have been 
overturned, and are even able to swim a short distance. The swiftness of 
the sea-snails is not always in proportion to the size of their foot, as the 
palelle creep but very slowly along on their broad disk. In some species, 
that remain fixed to the rock to which they first attach themselves, as small 
free-swimming larvae, the foot is naturally reduced to the state of an adhe- 
sive organ, 
Most of the Gasteropods are so heavily clothed, that they are necessarily 
confined to the rocky or sandy sea-bottom. The Janthina, however, has 
under its foot a vesicular organ, like a congeries of foam-bubbles, that 
serves as a buoy to support them at the surface of the water. When the 
sca is quiet, they appear in vast shoals on the surface, with their foot 
turned upwards; but as soon as the winds rufiie the ocean, they empty 
their air-cells and sink to the bottom, pouring out at the same time a dark 
red fluid, which, according to Lesson, furnished the celebrated purple of 
the ancients. The transparent shell is also of a beautiful violet color. 
The sea-snails inhabit different zones of depth; some live only within 
reach of the spring floods, and are therefore almost constantly out of the 
water ; others dwell a little lower, so as to be bathed at least by every flood ; 
