MOLLUSCA. 21 



in the sea, and some leap by rapidly closing their 

 shells, or by drawing in water and suddenly 

 forcing it out again. 



Teacher. When we consider the different 

 kinds of mollusca, I shall speak to you more 

 fully of their peculiar habits. Have you ever 

 observed when you have been on the sea-coast, 

 numerous shells clustered together on the rocks 

 and stones. 



Child. Yes, barnacles ; are they the animals 

 which never move from the spot to which they 

 are attached? 



Teacher. Yes, the barnacles, and many other 

 mollusca never quit the spot where their exist- 

 ence commenced. 



Child. How are they fixed to the spot ? 



Teacher. Some of them, as the barnacles, 

 are cemented to the rocks by the same substance 

 of which their shells are formed ; others have a 

 less permanent mode of adhesion, and fasten 

 themselves by means of a viscid liquid which 

 they discharge from glands in their bodies ; some 

 produce silky filaments, one extremity of which 

 remains in connexion with themselves, while the 

 other is fixed to marine substances, and thus an- 

 choring themselves, they float secure in the 

 ocean. The bundle of filaments which the ani- 

 mal draws from his body for this purpose is 

 called a byssw.* Do you know any other 



