ON MOLLUSCA. 191 
of reptation has no resemblance to that of reptiles : it is rather 
a kind of sliding of the foot, produced by extremely fine undu- 
lations of all the little longitudinal fasciculi which compose it, 
and which proceed in succession from the first to the last, each 
being alternately the point of resting, or the fixed point for the 
following. By this mode of locomotion, the animal touches one 
after another all the eminences, all the anfractuosities of the 
ground on which it moves; its advance is therefore in general 
very slow. Nevertheless, the species whose foot is broad, thick, 
extended, and has no shell to drag along, especially where the 
distinct contractile fibres have an evident fascicular direction, 
as in the limaces, helices, &c., can get on with a much greater 
degree of rapidity than one would be led to suppose from their 
first aspect. Others, on the contrary, such as the patelle 
and the haliotides, though possessed of a broad foot, creep 
so slowly, and so rarely change place, that some persons have 
erroneously believed that they never do so. They are capa- 
ble, besides, of adhering to a fixed point with very great force, 
through the viscosity of their foot and the vacancy which it 
can make, either altogether or in little fossettes. 
The hipponyces and some others remain fixed to the bodies 
on which they have fallen when born. Accordingly, their foot 
is scarcely muscular, and much resembles the horse-shoe mus- 
cle of the back, which serves as an attachment for the shell. 
The scyllz, whose foot is extremely narrow, and, as it were, 
canaliculate, cannot move but along the stalks and pedicles 
of plants, and always in a gliding manner. 
A tolerable number of species can also creep along the sur- 
face of the water, taking as a point d’appui a light stratum 
of this fluid. But then they are obliged to do it in an inverted 
position ; that is to say, the shell being under, and the inferior 
surface of the foot above. This is observable in the limnee, 
the planorbes, the paludine, glaucus, doris, thethys, &c. ‘The 
