PERILS OF THE OYSTER. 235 



worm bore through so hard a substance as an oyster- 

 shell ?" and the mode adopted is well worthy of notice. 

 There is reason to believe that sea-worms accomplish this 

 task by means of flinty teeth, which stud their ribbon- 

 shaped tongues. They must be actually beheld in order 

 to excite due admiration. We cannot construct a model 

 of them, even of a much larger size ; for how can we imi- 

 tate the life with which they are endowed, or the instinct 

 that regulates their movements ? These microscopic teeth 

 are beautiful objects ; withal constant in their shape, and 

 arranged in transverse rows on the tongue. 



Frequently the oyster will resist the invasion of the 

 enemy by depositing some pearly matter between its tender 

 body and the mouth of the invader, and thus compel him 

 to beat a retreat. But others are not so fortunate ; for in 

 the holes drilled by the sea-worms a preparation is often 

 made for the assaults of a parasitical sponge, which insinu- 

 ates itself and eats further than its predecessor into the 

 oyster, causing the softer parts of the shell to rot away, 

 and spreading through the whole substance of the oyster 

 like a dry rot in wood, until vitality is destroyed and its 

 loosened shell becomes detached and empty on the 

 waters. 



Such is the Cliona, of which Mr. Hancock has deter- 

 mined upwards of fifty species, all inhabiting more or less 

 diversified chambers in calcareous substances ; three or 

 four of which are found in the common ovster. Of these 



J 



twelve belong to the British seas ; the rest exist in various 

 parts of the world. They are most numerous in warm 

 climates ; none have been yet procured from the polar 



regions. 



On the coast of Northumberland the surface of almost 

 every piece of limestone near low- water mark is riddled by 



