104 



THE OPERCULUM. 



the part nearest the right hand is called the right side, 

 and that nearest the left hand the left side ; the part 

 facing him is called the front, and the reverse the 

 back. 



If a shell be so cut down the 

 middle that the inside may be seen, 

 a pillar will be observed, round which 

 the w^horls appear to wind ; this is 

 called the columella, from the Latin 

 for a little column ; the edge of the 

 mouth nearest it is called the colu- 

 mellar lip, and the one outside the 

 mouth the outer lip. 



The species of mollusca which 

 W'Ould suffer from the mouth being 

 open, have this organ provided with 

 a kind of lid, closing the entrance to the shell when 

 the animal retires into it : it is called operculum, the 

 Latin word for covering. The operculum for land 

 animals, such as snails, is temporary, and after a cer- 

 tain period falls off; but that with which a marine 

 shell is provided, is either a calcareous or horny sub- 

 stance, attached, not to the shell, but to the foot of the 

 animal, who draws it over his mouth when he pleases 

 to recede into his dwelling. 



