120 OYSTERS, AND ALL ABOUT THEM. 



layers of shell mould themselves upon it ; and the animal 

 acquires the asymmetry characteristic of the adult. 



" Oysters are gregarious, in consequence of the vast 

 multitude of locomotive larvae which are set free simul- 

 taneously ; and which, being subjected to the same influ- 

 ences, tend to settle about the same time in the area to 

 which the swarm drifts. Millions of oysters are thus 

 aggregated together over stretches of the bottom of the 

 sea, at depths of from one or two to twenty or more 

 fathoms, and constitute what are known as oyster beds. 



" Although oysters live and grow well enough in estu- 

 aries, in which the salinity of the water undergoes large 

 variations, according to the state of the tide and the volume 

 of fresh water that is poured in, yet they do not flourish 

 permanently and breed freely in water with less than 3 per 

 cent, of saline constituents. Thus the Baltic is, at present, 

 unfit for their support, and the east coast of Schleswig, 

 washed by its brackish waters, is devoid of oysters, while 

 certain parts of the west coast are famous for their oyster 

 beds. Gravel, stones, and dead shell commonly known 

 as ' cultch ' -form the most favourable bottom, as they 

 facilitate the attachment of the young. Disturbed muddy 

 bottoms, on the other hand, are fatal because turbid water, 

 laden with coarse sediment and entering the infra-bran- 

 chial cavity, particles of mud, too large to be moved by 

 the cilia, lodge upon the gills, and, gradually obstructing 

 the current, interfere with the primary functions of feeding 

 and breathing to such an extent as to injure, or even to 

 destroy the animal. But it is a curious fact, that even 

 where a large extent of sea-bottom presents apparently the 

 same conditions, ovster beds occur in some localities and 



j 



not in others." (0} 



(o) " Oysters and the Oyster Question/' Professor T. H. Huxley. 



