SHELLS AND MOLLUSCOUS ANIMALS. 175 



ing to their conditions ; thus, those which live on 

 sea-weeds have horny jaws, or minute teeth : and 

 those which, like the whelk, feed on flesh, have 

 generally a long pliable tube, armed with teeth, 

 by which they pierce the shells of their neigh- 

 bours, and draw out their juices ; while such as 

 are destined to be fed only by the minute living 

 things of the water, are destitute of these organs, 

 and have mouths formed only of a little aperture. 



Shells are of two kinds, porcelainous and mem- 

 branous. Some are, like the cowries, which are 

 brought from far distant seas to ornament our 

 rooms, of a compact texture, resembling porcelain, 

 and the surface is beautifully enamelled and va- 

 riously tinted. The membranous shells, like that 

 of the oyster, are formed of layers, composed of 

 mother-of-pearl, and are often covered with a 

 strong epidermis, or skin. All shells consist of 

 carbonate of lime, mingled with animal gelatine, 

 but the porcelainous kinds contain a far less pro- 

 portion of animal matter than exists in the mem- 

 branous shells. If the carbonate of lime contained 

 in the last-named shells be subjected to acid, it 

 will dissolve, and leave nothing but the thin mem- 

 brane from which they take their name ; while the 

 more brittle porcelainous shell has its substances 

 more equally blended in a crystalline arrange- 

 ment. 



Beginning at the lowest orders of molluscous 

 animals, and gradually rising to the highest, we 

 must select some of the common objects on our 

 shores belonging to the class Cirrliopoda of the 

 naturalist, the barnacles, which every sailor 

 knows by their familiar name, and upon which 

 most of us have often looked. The commonest 



