MOLLUSCA. 15 



fixes itself upon the rocks, is very simple, 

 though very curious. It forms within its 

 shell, a vacuum, that is, a space free from air, 

 derived from Latin, vacuws, empty. By filling up 

 the hollow of the shell with the fleshy substance 

 of its body, it expels the air, and having fastened 

 its body to the rock by a viscid substance, con- 

 tracts itself in the centre, thus leaving a space 

 within the shell entirely free from air. As there 

 then is no internal expansion of this fluid, 

 the pressure of the external atmosphere meets 

 with no resistance, and in consequence acts with 

 so much force upon the shell, that it cannot be 

 removed from its situation without great effort. 

 Repeat to me now the different means of attach- 

 ment used by the mollusca. 



Child. Some fix themselves by silky fila- 

 ments called a byssus, some by a viscid cement, 

 others by forming a vacuum, and others attach 

 themselves to the rocks by the same substance 

 of which their shells are made. 



Teacher. The shells which by any of these 

 means are rendered stationary, are called fixed 

 shells, the others free shells. Does anything 

 strike you with regard to these two kinds of 

 mollusca ? 



Child. The inhabitants of the fixed shells 

 must be badly off. They cannot procure nour- 

 ishment. 



Teacher. They cannot indeed go in search 

 of it, but the continued motion of the waves, or 

 the flowing of the tide brings a fresh supply of 



