ON MOLLUSCA. 183 
All the species of cephalous mollusca are eminently gaste- 
ropods. It is not thus with the conchyliferous species: they 
are not necessarily trachelipods, though this be most com- 
monly the case. But what they present in common is, that 
the shell is in muscular communication with this foot, so that 
the latter can be drawn in more or less deeply, as well as the 
head itself, by a muscular bundle called the muscle of the 
columella, because, in the spiral shells, it is to that part it is 
attached. ‘The disposition of this bundle of muscles differs 
much, according to the form of the foot, and especially accord- 
ing to that of the shell. ‘Thus, when the latter is simply 
covering, and not spiral, as in the patelloides, the bundle in 
its dorsal attachment forms a sort of horse-shoe, open in front 
or behind, and the termination of the foot extends almost 
through its entire circumference. In those whose shell is a 
turriculated spiral, the common bundle is pointed at the colu- 
mella, and is carried more or less obliquely from the hinder 
part to the front, and from top to bottom, towards the middle 
of the foot, so that by its contraction it almost bends the latter 
in two in withdrawing it within the shell. In the species 
where the rolling is lateral, as in the porcelaines, it is, on the 
contrary, a broad muscular band, which is longitudinally in- 
serted at the columella, and which terminates at the foot, in 
such a manner as to bend the latter in its length, to draw it 
within the shell. 
We may also regard as making a part of this muscular bun- 
dle the more or less considerable muscles, which are carried 
forward to proceed to the tentacular and ocular appendages, 
where these organs can be withdrawn internally, as is the case 
with the limacines. They penetrate, in fact, into these ten- 
tacula, and proceed as far as their extiemity, surrounding the 
nerve, which also repairs to the organ. 
In the species which have these sorts of tentacula, and 
have not a crustaceous shell, and whose foot has consequently 
