278 GASTEROPODA. 



rately fits the orifice of the shell, to which it serves 

 as a door, when the animal withdraws into its habi- 

 tation. 



FlG. 215. THE WHELK, SHOWING ITS OPERCCLUM. 



All the Pectinibranchiate Mollusks have two ten- 

 tacles and two eyes, sometimes supported on special 

 foot-stalks. Their mouth is in the form of a tube or 

 proboscis, capable of being protruded by a very 

 peculiar mechanism, and furnished at its extremity 

 with a kind of file, by the aid of which they bore 

 through the shells of other Mollusca, notwithstanding 

 the massiveness of the defensive armour of their 

 victims. Their eggs are very numerous, and are 

 generally enclosed in cases of complicated form and 

 very curious structure. 



The beauty, and more especially the rarity, of 

 the shells of many species have often caused them to 

 bear a very high adventitious value. The elegant 

 Chinese shell, known as the Royal Staircase or 

 Wentle-trap, derived its specific name (Scalaria pre- 

 tiosa) from the high price at which large and fine 

 specimens were sold. One was purchased in France 



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