290 THE INHABITANTS OF THE SEA. 
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 Olive, 
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 Strombidee and Rostellarize place their power- 
ful 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 remark- 
able: 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 ad- 
vanced 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. 
Tartishieconinania. The Ianthinze, 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. 
Pteroceras scorpio. 
Oliva hispidula. 
Strombus pes pelicani. 
