SHELLS. 49 



within it. This lid is called the operculum, a 

 latin word, signifying a covering. The shells 

 which are furnished with this appendage, are said 

 to be operculated. Did you ever observe any- 

 thing 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 decide, I fall. The operculum 

 of the marine shell is of a very different nature. 

 It is either a calcareous or a horny substance, 

 and is permanent, but 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. Look at all the shells and observe 

 whether that is invariably its place. 



Child. No, here is one shell in 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 y 

 the Latin for the right hand. The sinistral shells 

 are also said to be reversed, because the whorls 

 proceed in a direction contrary to their usual 

 E 



