THE IMPENETRABLE SEA 



existence they sink to the sea-bed, or on to the netting, 

 brush or lattice-work provided by man as he prepares to 

 take advantage of their forthcoming change to a state of 

 immobihty. 



Oysters are distributed as the result of their free exist- 

 ence in infancy, for drifting spat (as the larvae are 

 called) may travel long distances. No one knows how 

 long oysters may live if undisturbed. Some authorities 

 say they can live for as long as thirty years. But when 

 artificially reared they are allowed only four or five years 

 of life before being collected for human consumption. 



The oyster is a highly developed creature. It has a 

 liver, intestines, and a heart with a blood-circulating 

 system (although the blood is colourless) and it even has 

 a complicated nervous system and a brain. None of these 

 organs resemble our own, but they are as described. 

 Although the common scallop has eyes — a hundred of 

 them, each with its lens, retina and optic nerve — the 

 oyster is eyeless. Its enemies include rays, starfish, boring 

 sponges, certain marine leeches, octopuses, and those 

 most deadly foes of the oyster the bloodthirsty insatiable 

 Dog whelks and oyster tingles. 



Although these are burdened with a heavy shell they 

 roam about seeking whom they may devour. They are 

 ruthless housebreakers and nature has obligingly pro- 

 vided them with a file, to rasp its way into the shells of 

 other organisms. The file is called a radula, and is a 

 ribbon of closely-set teeth. 



The surf-scoter, a bird that lives in the northern 

 oceans, is not provided with a beak suitable for opening 

 oyster-shells, so it simply swallows them whole. They 

 open inside the bird's gizzard, in which plenty of stones 

 are lodged to grind the shells to pieces. 



In Jamaica the lower branches of mangrove trees are 

 often covered by water at high tide. Oysters suspend 

 themselves from the branches, so that it would be true 

 to say that they "grow on trees" in that country. 



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