SUB-DIVISION OF THE MOLLUSCA. 505 



disposed in a symmetrical series, as in the vertebrate and 

 articulate animals. 



The skin of the mollusca, always soft and viscous, often 

 forms folds which envelope more or less completely the body ; 

 and this disposition has induced zoologists to give the name 

 of mantle to that portion of the integument generally fur- 

 nishing these expansions. This mantle is often entirely free, 

 forming two large veils or coverings concealing all the rest 

 of the animal, or these two laminse unite so as to form a 

 tube ; but at other times it consists only of a kind of dorsal 

 disc, of which the edges alone are free or surround the body 

 more exactly under the form of a sac. 



594. In general the soft skin is protected by a kind of 

 stony or hard cuirass, called shell. It is a tissue which has 

 some analogy with that of the epidermis, which constitutes 

 this envelope. The follicles lodged generally in the edges of 

 the mantle, deposit on its surface a half-horny matter, mixed 

 with carbonate of lime in greater or less proportions, and is 

 moulded over the subjacent parts, and next solidifies. The 

 lamina thus formed thickens and grows by the successive 

 deposit of new matter. Its surface is not stony, but resembles 

 a kind of epidermis, and is known by the name of drap marin, 

 or sea- cloth ; sometimes it preserves a horny consistence 

 throughout its whole thickness, but in general the proportion 

 of carbonate of lime which it encloses gives to it a stony 

 hardness. Its inner surface is frequently more dense than 

 the rest, and presents a peculiar structure rendering it glassy, 

 lustrous, and pearly. Sometimes the shell remains always 

 enclosed in the thickness of the skin of the animal; but 

 generally it is external, and even passes beyond the edges of 

 the mantle, so as to furnish to the animal perfect shelter. The 

 name of naked mollusca has been given to those which are 

 without shells, or which have only an interior one, and the 

 name of conchifera to those in which the shell is visible 

 externally. 



The manner in which this shell grows may be readily un- 

 derstood. If we examine the shell of an oyster, for example, 

 it will be found to be composed of a number of superimposed 

 laminae, which can be separated by means of heat. These 

 plates have been formed successively by the mantle of the 

 animal, which they cover, and consequently the most external 

 is the oldest, or first formed, and the smallest; each new 

 plate deposited passes beyond the plate situated above it, 



