CURIOUS DEFENCE. 



123 



mollusca. When the animal which inhabits a spiral 

 shell retires within it, the only part of its body exposed 

 to injury is that which is situated at the mouth of its 

 dwelling. But the Creator has not designed that it 

 should be without defen(*e. In order to its protection 

 it is, in many instances, prepared to construct a separate 

 plate of shell, adapted to the aperture, and called an 

 operculum. This engraving exhibits 

 the lines which appear on the inner 

 side of that of the turbo, and which 

 show the successive deposits by which 

 it has been formed. 



A clausium is another kind of cover- 

 ing, and consists of a thin spiral plate of shell at- 

 tached to the columella by an elastic spring, by which 

 the plate is retracted when the animal retires into its 

 shell. It thus exactly corresponds with a door, open- 

 ing and closing the entrance as occasion requires. 



An epiphragma is a partition made merely for tem- 

 porary use. On the approach of winter, the helia, or 

 garden snail, prepares itself for passing that season in a 

 torpid state, choosing a safe retreat, retiring completely 

 within its shell, and then forming the defence just de- 

 scribed, of which the following engraving is a repre- 

 sentation of its outer surface. 



