162 MOLLUSC A DIAGRAM 7. 



All the bivalve molluscs do not live attached to rocks like the 

 oyster. The cockle and many others move from cue place to 

 another. So does the fresh-water mussel, which is common in 

 streams and ponds. The oysters, on the contrary, always live 

 fixed in the place where they have attached themselves after 

 their birth. The true sea-mussels anchor themselves by means 

 of filaments which they can reproduce when they have been torn. 

 These filaments are called collectively the byssiis. 



Pearls. When eating oysters, we occasionally find small pearls 

 in them, which are sometimes quite round, and sometimes of a 

 less regular shape ; they are of no value ; but true pearls are also 

 found iii a large kind of oyster called the pearl-oyster, which is 

 only found in tropical seas, especially on the coast of Ceylon. 

 These pearl-oysters are always found at great depths, and are only 

 procured by very skilful divers. In order to sink quicker to 

 the bottom, they put their feet in a loop fastened to a stone 

 which pulls them down ; they remove the shells which they find 

 with a knife, and put them in a basket fastened to a cord ; and 

 then rise to the surface to breathe. True pearls are always very 

 hard, while imitation pearls made of glass are crashed by the 

 slightest force. 



The shell which produces pearls is itself employed in the arts 

 under the name of mother-of-pearl. 



Cuttle-fish. We sometimes pick up a flat oval white object on 

 the sea-shore, which is hard on one side, and friable on the other, 

 and which is very light. This is called the 

 bone of the cuttle-fish ; and is used to make 

 tooth powder. The bone of some species is 

 long, and is called the sectrpen. The cuttle- 

 fish are molluscs which have no shell except 

 this, which is found under the skin of their 

 back. The cuttle-fish have arms round their 

 head, provided with a number of small suckers 



which adhere to any object which they wish 

 Cuttle-fish. . -, . . . . ,. .... 



to seize. Ihc mouth is placed in the middle 



