136 SCIENCE FROM AN EASY CHAIR 



thus on to bad ground ; a vast number have already 

 been eaten by young fishes and shrimps. Those which 

 are lucky enough to fall on to some- 

 thing hard stones, rocks, old oyster- 

 shells, or the shells of living oysters 

 become cemented to those hard sub- 

 stances by the new shelly substance 

 formed by the growing edge of the 

 lowermost of their little shells, which 

 now spread out, lose their cockle-like 

 shape as they grow, and become, the 

 one (the left by which it is fixed) large, 

 deep, and bossed, the other flat. The 

 little oysters are only one-fortieth of 

 an inch in diameter when first they 

 become fixed, but they grow rapidly, 

 feeding in the same way as their 

 parents. Vast numbers are eaten by 

 other animals. In some localities in 



head, with its tuft 

 of cilia projecting 

 from between the 

 two shells, / and r. 



FIG. 34. F r e e- 



swimming young 

 oyster or oyster- 



larva, showing the two years, in others in three years, 

 they have grown to a couple of inches 

 in length, and now produce in the 

 summer breeding season a certain 

 quantity of eggs and sperm to start 

 new generations. The oyster continues to grow, and at 

 five to seven years of age is in full vigour and maturity ; 

 at ten years of age it produces few eggs, or sperm-cells ; 

 and in the course of another year or so, under natural 

 conditions, dies. 



Enormous as is the output of young by a single 

 oyster amounting to something like a million a year 

 in probably four or five successive years yet it must be 

 remembered that on the whole, taking all the various 

 oyster-beds into account, some of which increase whilst 

 others dwindle or actually die out altogether, there is no 



