756 MOLLUSCA. 



snoul 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 oesophagus, 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 loligo ; in 

 others, they are small and imperfect, and are borne at the extremity of a 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 

 moUusca, 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- 

 tle. Many species are naked ; but by far the greater number are enveloped 

 in a 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 sometimes 

 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 shell is extreme- 

 ly varied. In general, it is spirally 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, 

 round which the shell is contorted. When the columella is hollow, its 

 opening is called the umbilicus. 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 ; 



