MOLLUSCA. 15 



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 attachment 

 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 calledyfoec? 

 shells, and those inhabited by animals that can 

 move them, free shells. Does any thing 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 

 water, swarming with the little animals upon 



