22 LESSON I. 



shells that are with difficulty removed from the 

 rocks ? 



Child. Yes, the limpet. 



Teacher. The manner in which the animal 

 you mention fixes itself upon 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, free. By filling up 

 the hollow of its 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 re- 

 moved 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 any thing 

 strike you with regard to these two kinds 01 

 mollusca ? 



Child. The inhabitants of the fixed shells 

 must be badly ofT. They cannot procure nou^ 

 rishment. 



