Descriptions of Specwtens 591 



kind of sole to the animal's " foot." To this substance the 

 term Opevcitlum is given. 



Opercula vary much in size and thickness in different 

 animals, and are sometimes ornamented by colours, and by a 

 spiral pattern. The bivalve Mollusca have no opercula, nor 

 do they need them, since they are completely protected by 

 the second or flatter shell. It may be plausibly supposed 

 that the opercula in univalves represent this second shell, 

 from which they differ, however, in not being in any way 

 hinged to the larger one. Some opercula are oval, but more 

 usually they are round. They do not look like shells, and 

 those which are coloured might be taken for discs of earthen- 

 ware. Some of them are used as ornaments, but the majority 

 present no particular beauty. 



SHELLS OF A PHOLAS. 



These shells are so thin and delicate that they are often 

 called " paper-shells." For their protection the molluscs 

 inhabiting them bore holes in rocks in which they lodge. 

 Although so apparently fragile, they yet use the thicker end 

 of their shells as a rasp, and by a rotary motion file away 

 the rock until a hole is made. This is their only method 

 of boring. It is a mistake to suppose that they secrete any 

 acid which dissolves the stone. They bore into wood as 

 well as stone. 



