CONSERVATION OF DEEP SEA BEDS. 1039 



ditions, a knowledge of the elevations and depressions of 

 the sea-bottom are of importance, as affecting the distri- 

 bution of oysters, and the directions in which the banks 

 extend. 



Besides affecting pressure, depth influences the amount 

 of light, aeration of the water, and its temperature. 



Oysters dredged up from greater depths than twenty- 

 five fathoms are generally thin and coarse ; thin, because 

 the amount of alluvial deposit containing certain kinds of 

 infusoria necessary for their food is mixed with so much 

 sea-water that their food is weak and scarce ; and coarse, 

 because they have to feed on foraminifera, which has much 

 the same effect on oysters as a meat diet has on pigs ! 



The lar^e shallows in the estuarv of the Thames, 



O - 



where the banks are just covered at low-water neap-tides, 

 the Kentish flats, &c., become unduly heated during the 

 long summer days, and there appears to be no reason to 

 doubt that this is the cause of the Burnham, Roach, and 

 other rivers thereabout, being very favourably situated for 

 the production and deposit of spat, the tide as it rises 

 bringing in the heated water and raising the temperature 

 of these rivers. 



TIDES AND CURRENTS 



Exercise great effect on oyster culture by running away 

 with the spat until it dies, or is deposited in localities 

 unfavourable to its continued existence. Mr. Buckland 

 said that the young oyster in its first stage of life moved 

 easily about in the water " as thistle-down in a field is 

 carried to and fro by light airs." He also thought that the 

 cilia enabled the spat to guide themselves to a certain 

 extent, but of course they could not make headway against 



