182 SUPPLEMENT 
although the muscular stratum which doubles it does not form 
distinct muscles, presents differences only in the thickness of 
this stratum in different points of its circumference. Thus, 
sometimes, this thickness is pretty nearly the same, whence 
results a sort of sac, as in sepia, &c.; but more frequently it 
is much greater at the lower part of the body, where even the 
fibres, though longitudinal, experience frequent intersections, 
and a sort of muscular disk, more or less thick, is thus formed, 
to which the name of foot is given. : 
In a number of cases, this sort of foot extends through the 
whole length of the body, or rather of the mass of the viscera 
which is above, and forms a kind of sole, somewhat varying 
in form, by the assistance of which the animal creeps, and 
which occupies its entire belly. From this conformation comes 
the denominations of repentia, or gasteropods, which have 
been given to the limaces, and neighbouring genera. 
In some species the visceral mass comes forth, as it were, 
beyond the mass of the body, being rolled more or less spirally ; 
and the foot, containing no more viscera in its posterior part, 
where it is free, appears to be no further attached to the body, 
except before or behind the head, at the part which might be 
regarded as the neck; from which arrangement these mollusca 
have obtained the name of trachelipods. Most of the cepha- 
lous conchyliferous mollusca are thus formed, especially when 
the shell is strongly spiral and rolled. 
The proportional size, and even the form of this foot, vary 
much, sometimes even in genera pretty much approximating 
to each other. Thus it is almost circular in the patella, oval 
and very thick in the haliotides, rounded in front, and slender 
on the sides in the turbines, buccina, &c. It is auriculated on 
each side in some. In a great number of genera of the order 
cyclostomata, it is cut by a transverse furrow at its anterior 
edge. The foot of the auricula pecten is divided into two heels 
by a broad transverse furrow. 
