THE OYSTER. 27 



is left. It has all gone back into a form which makes 

 it available as oyster food, and it soon begins its trans- 

 formation into new oyster shells. In this way the 

 oysters obtain some of their lime directly without 

 being compelled to draw on the inland beds of ancient 

 fossils, and if all the shells could be returned to the 

 beds, this source of supply would be greatly increased. 

 The difference between the right and the left shells 

 of the oyster has a very profound significance, for in 

 science nothing is trivial or unimportant. Most of 

 the near relations of the oyster, like the clam and the 

 fresh-water mussel, have the two sides of the body, and 

 the two shells, alike. These animals are not fastened 

 nor stationary like the oyster. They move from place 

 to place in search of food, and their line of locomotion 

 lies in the plane which divides the body into halves. 

 They are erect and bilaterally symmetrical like other 

 locomotor animals, such as the horse, the fish, the 

 butterfly and the crab. The full-grown oyster has no 

 locomotor power and it lies on its left side, but in the 

 early part of its life it is very active, and is then bilat- 

 erally symmetrical like the clam. When it ceases 

 its wanderings and settles down for life, it topples 

 over, falls on its left side, and fastens itself by its left 

 shell, which soon grows deep and spoon-shaped, while 

 the right becomes a flat, movable lid. The body, 

 which was originally symmetrical, becomes dis- 

 torted or twisted to fit the difference in the shells, 

 and naturalists see in the fact that the locomotor 

 relations of the oyster are symmetrical through life, 

 while the oyster loses its symmetry as soon as it 

 settles down, one of the proofs that it is descended 



