330 MANTLE AND SHELL OF MOLLUSCA. 



inclosed in a spongy elastic skin, with which muscular fibres are 

 interwoven ; this is termed the mantle. This envelope is fre- 

 quently not applied closely to the surface of the organs contained 

 in it ; especially among the lower classes, in which the space 

 thus left constitutes a respiratory chamber. It has apertures for 

 the admission and exit of the surrounding water, to effect the 

 aeration of the blood ; and, when the mouth is not capable of 

 being projected beyond it, the same current furnishes the supply 

 of food. These apertures are sometimes extended, for particular 

 purposes, into proboscis-like tubes (Fig. 697). Where the head 

 is capable of being protruded, there is usually an opening in the 

 mantle for the purpose ; and another for the foot, where it 

 exists as a separate organ. 



941. The mantle is chiefly interesting, as being the portion of 

 the body alone concerned in the formation of the shell. Some- 

 times this envelope is secreted from nearly the whole surface ; 

 sometimes from only a small part of it. As the same general 

 statements in regard to the constitution of this body will apply 

 to all the Mollusca, its origin and essential characters may 

 advantageously be described, previously to the more detailed 

 account, which will be given of the several classes, in which its 

 leading peculiarities of form will be noticed as they occnr. Shells 

 are formed, like bones, of a combination of earthy and animal 

 matter. The former consists entirely of carbonate of lime, 

 which is usually deposited in a crystalline condition. The 

 latter is composed of layers of membrane, alternating with the 

 mineral matter; and of cells inclosing it. The cellular structure, 

 which seems to be of the nature of Epithelium (ANIM. PHYSIOL. 

 39), is generally found upon the surface. If the carbonate 

 of lime be removed by the slow action of a weak acid, the 

 animal matter will remain, sometimes in the form of a continuous 

 membrane, but sometimes only as a flaky mass, easily shaken 

 into separate portions. The shell is most solid and massive in 

 those species which lead an inactive life ; and it attains greater 

 weight in the Conchifera, in which every species forms a stony 

 covering, than in the Gasteropoda, whose powers of locomotion 

 are somewhat greater, and in which the shell is often absent, 



