ANIMALS FOR. 57 



(Alasmodon margaritiferus), Fig. 23. It is in the 

 shells of this mussel that pearls are most frequently 



FIG. 23. 



found : they occur occasionally in all the larger 

 forms of mussels. In the inside of all shells a 

 beautiful lustrous substance is found, called nacre ; 

 if this, from tiny cause, concretes together, it 

 forms a pearl. Pearls may be formed artificially, 

 by introducing some irritating substance between 

 the mantle and shell. Linnseus, the great natu- 

 ralist, was aware of this, and thought to make his 

 fortune by the discovery. British pearls were 

 known in the time of the Romans, and it is said 

 the reputation of these pearls was one of the minor 

 inducements that brought Csesar and his soldiers to 

 our shores. They are still procured from the mussel 

 in the river Conway, and although inferior in size 

 and colour to those obtained from the pearl oysters, 

 are introduced into the less- prominent parts of the 

 ornaments which are manufactured from them. 



3. Radiata. 



To this class of animals the various forms of 

 INFUSORIAL ANIMALCULES (Infusoria) belong. 

 Under all circumstances they will generally be 



