MOLLUSKS. i(X) 



in their descent to the bottom of the sea by a heavy weight tied 

 to their feet. They rapidly gather all the Pearl Oysters in their 

 way into a basket, and when in want of air, give a signal to their 

 friends above, who draw them to the surface by a rope. The 

 oysters are then left to putrefy for some weeks, when they are 

 carefully washed, and the pearls extracted. 



The Sea Mussels are usually fixed where the tide leaves 

 them alternately wet and dry, and it is worthy of notice that those 

 " shell-fish" which are exposed to variations of this kind are en- 

 abled to close their shells so firmly as to prevent any evaporation. 

 One species is extensively used as an article of food. 



The river mussels occasionally produce pearls of some value. 

 The nacre of these mussels is of a beautiful azure blue. 



The Bernicle At first sight, the Bernicle bears a close 



resemblance to a mussel-shell fixed to a long stem. On a closer 

 examination, however, the difference is at once apparent. The 

 shell is in fact composed of five pieces, and through the aperture 

 of the shell are thrust two rows of arms, or " cirrhi," as they are 

 more properly called. These cirrhi serve to entangle the small 

 Crustacea or moUusks which pass near their sphere of action, and 

 which are then carried to the mouth and speedily devoured. 



The Bernicle is always feund adhering to some larger ob- 

 ject, usually floating wood, and is very common on the hulls of 

 ships. Although the perfect animal is permanently fixed, it has 

 been discovered that the young are free and capable of locomotion ; 

 nor is it until a week or two has passed, thkt they finally settle 

 themselves. 



S5 



