162 MOLLUSCA DIAGRAM 7 



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

 oyster. The cockle and many others move from one 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 byssus. 



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 j but true pearls are also 

 found in 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 crushed by the slightest 

 force. 



The shell which produces pearls is itself employed in the arts 

 under the name of mother-of-pearl. 



Cuttlefish. 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 sea-pen. The cuttle- 

 fish are molluscs which have no shell except 

 this, which is found under the skin of their 

 back. The cuttlefish have arms round their 

 head, provided with a number of small suckers 

 which adhere to any object which they wish 

 Cuttlefish. to seize. The mouth is placed in the middle 



