SHELLS. 49 



and closes the entrance to the shell, when the 

 animal retires within it. This lid is called the 

 operculum, a latin word, signifying a covering. 

 The shells which are furnished with this appen- 

 dage, are said to be operculated. Did you ever 

 observe anything at all like it in the snails ? 



Child. The entrance to the shell is guarded 

 by a thin covering during the winter. 



Teacher. The animals belonging to land 

 shells, as for instance snails, protect themselves 

 from the inclemency of winter, by forming a 

 temporary covering, which adheres to the sides 

 of the shell, and is deciduous, that is, falling 

 after a certain period, from Latin decido, I 

 fall. The operculum of the marine shell is 

 of a very different nature. It is either a calca- 

 reous or horny substance, is permanent, and not 

 attached to the shell, but to the foot of the 

 animal, who draws it over the mouth when he 

 recedes into his abode. Now tell me the position 

 of the mouth. 



Child. It is on the right side of the shell. 



Teacher. Observe whether that is invariably 

 its place. 



Child. No, here is one shell it which it is on 

 the left side. 



Teacher. This is the case in some few species, 

 and the shell is then said to be sinistral, a word 

 derived from the Latin sinistra, the left hand. 

 The others are called dextral shells, from dextra 

 the Latin for the right hand. The sinistral shells 

 are also said to be reversed, because the whorls 

 E 



