MOLLUSCA. II 



sucking their flesh. Those which have the parrot beak are 

 also carnivorous. Those which have a snout and two jaws, 

 of which one at least is furnished with small teeth, are herbi- 

 vorous or frugivorous. 



The intestinal canal consists of an internal mucous membrane, 

 commonly 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 oasophagus, 

 and sometimes that organ is very large and wide. The sto- 

 mach is frequently simple, but also divided into several cavities 

 or cells. The liver, composed of lobes and lobules, is situated 

 more or less behind the stomach, very frequently at the poste- 

 rior part of the body. The ducts unite into three or four ca- 

 nals, which empty themselves 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 Mollusca is pecu- 

 liarly soft and spongy, and from its connection with the sub- 

 jacent muscles slightly contractile. It is smooth or tubercu- 

 lar, and generally secretes a large quantity of mucus. It ob- 

 tains the name of mantle. 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 so- 

 lid 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 pro- 

 duced by the Mollusca, there are therefore some which are truly 

 internal, not appearing at all externally ; 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 ex- 

 tremely varied. In general it is spirally convoluted. The prin- 



