THE OYSTER. 8 1 



left to themselves the rocks remain sharply defined. 

 What is the reason for this sharp limitation of a 

 natural bed? Those who know the oyster only in its 

 adult condition may believe that it is due to the ab- 

 sence of power of locomotion, and may hold that the 

 young oysters grew up among the old ones, just as 

 young oak trees grow up where the acorns fall from 

 the branches. This cannot be the true explanation, 

 for the young oysters are swimming animals, and they 

 are discharged into the water in countless numbers, to 

 be swept away to great distances by the currents. As 

 they are too small to be seen at this time without a 

 microscope, it is impossible to trace their wanderings 

 directly, but it is possible to show indirectly that they 

 are carried to great distances, and that the water for 

 miles around the natural bed is full of them. They 

 serve as food for other marine animals, and when the 

 contents of the stomachs of these animals are carefully 

 examined with a microscope, the shells of the little 

 oysters are often found in abundance. While examin- 

 ing the contents of the stomach of lingula in this way 

 I have found hundreds of the shells of the young oys- 

 ters in the swimming stage of growth, although the 

 specimens of lingula were captured several miles from 

 the nearest oyster-bed. As lingula is a fixed animal, 

 the oysters must have been brought to the spot where 

 the specimens were found, and as lingula has no means 

 of capturing its food, and subsists upon what is swept 

 within its reach by the water, the presence of so many 

 inside its stomach shows that the water must have con- 

 tained great numbers of them. 



It is^clear, then, that the sharp limitation of the area 



