118 



BOYS AND GIRLS IN BIOLOGY. 



the holy water in the Church of Saint-Sulpice. Some of 

 this family, living in the rivers of Scotland and Wales, 

 and in the Persian Gulf, produce small, hard, shining, 

 round, or egg-shaped bodies. Some of these lie on the 

 inside of the shell, but the prettiest and largest are 

 found inside the mantle near the straps, or adductor 

 muscles. They are so bright and beautiful, that they 

 are called jewels. You have often seen them and know 

 their name, but perhaps you have never thought where 

 they came from. People used to tell all sorts of stories 

 about them. As wise a man as Pliny thought they 

 were made out of the dew of heaven ; others said they 

 were drops of pure rain, which the mussel opened his 

 shell to receive. But science teaches that the heavenly 

 pearls have a very earthly origin. The animal makes 

 them by trying to heal a wound in his body, or to rid 

 himself of a foreign substance that gets into his shell, 

 such as a grain of sand. It is said that the pearl fish- 

 ers sometimes catch the mussels, and wound them by 

 pricking them with needles, and then throw them back 

 into the water. The poor creature throws out a sub- 

 stance to heal its wound, and this substance forms the 

 pearl. Some of them seem to be made of the little 

 eggs which are left in the mussel's body ; thus you see 

 to what a very useful family our friend belongs. Be- 

 sides this power to make pearls, many of them are de- 



