756 MOLLUSCA. 
snout and two jaws, of which one, at least, is furnished with small teeth, 
are herbivorous or frugivorous. 
The intestinal canal consists of an internal mucous membrane, common- 
ly forming longitudinal folds, and a more or less distinct muscular layer 
It varies much in respect to its direction and enlargement. Sometimes 
there is a long and narrow ceesophagus, and sometimes that organ is very 
large and wide. The stomach is frequently simple, but also divided into 
several cavities or cells. The liver, composed of lobes and lobules, is situat- 
ed more or less behind the stomach, very frequently at the posterior part of 
the body. The ducts unite into three or four canals, which empty them- 
selves into the stomach or intestine. The intestinal canal varies still more 
than the stomach in its diameter, the number and form of its circumvolutions, 
in its direction, and in the point at which it terminates. The organs of 
vision are largely developed in certain species, as in the genus Joligo ; in 
others, they ‘are small and imperfect, and are borne at the extremity ofa sort 
of tentaculum, or are sessile. The sense is in general very obscure, and in 
many species can scarcely be said to exist. The senses of hearing, of smell, 
and taste, are equally obtuse. The skin which envelopes the body of the 
mollusca, is peculiarly soft and spongy, and from its connection with the 
subjacent muscles, slightly contractile. It is smooth or tubercular, and 
generally secretes a large quantity of mucus. It obtains the name of man- 
tie. Many species are naked ; but by far the greater number are enveloped 
in 2 calcareous covering, named the shell. . Of those which are naked, some 
are entirely soft in all their parts; while others contain internally, one or 
more solid parts, which are sometimes merely cartilaginous or horny, or 
cretaceous, and lamellar, without being really conchyliform, and sometines 
constitute a true internal shell. Of the shells produced by the mollusca, 
there are, therefore, some which are truly internal, not appearing at all ex- 
ternally ; in others, the shell is disclosed, in part,.at the posterior extremity 
of the animal; while in a great portion, the shell is entirely external, and 
envelopes or covers the animal. The form of this external shellis extreme- 
ly varied. In general, it isspirally convoluted. The principal parts which 
it presents, are the aperture or mouth, consisting of an inner or columellar 
lip, and an outer lip; the body of the shell consisting of the last, and generally 
tumid turn, or whorl; the spine, formed of the convolutions, which are only 
in part seen, because enveloped by the last turn; and the columella, or axis, 
tound which the shell is contorted. When the columella is hollow, its 
opening is called the wmbilicus. The shell consists of a mixture of calca- 
reous matter, (carbonate of lime,) and gelatinous matter. This is frequently 
covered externally by a thin layer of the latter substance, forming what is 
called the epidermis. It exhibits a great variety of coloring. The form of 
the shell indicates that of the animal which inhabits it; and is used as fur- 
nishing the generic characters, the structure of the animals of this class not 
being generally known. It also furnishes many of the specific characters; 
