376 THE ZOOLOGIST. 



likely to increase the salinity to an undesirable degree, and on 

 the other hand, its presence enables the fattening ground to be 

 identical with or contiguous to the rearing-ground, thereby 

 effecting economy of labour. For fattening purposes fresh water 

 is indispensable. The density of the most suitable mixture 

 appears to be about 1014. Positions then, where there are 

 surface currents or springs of fresh water, or which are near the 

 mouth of a river or canal, are, if other requisites are present, the 

 most eligible sites for rearing and fattening grounds. There 

 should be daily tidal changes of water; and the stronger the 

 current the better for the Oysters, as they thereby receive more 

 abundant nutriment. The beneficial effect of strong currents is 

 very noticeable at Thau. The water need not be limpid or clear. 

 Turbidity is not injurious, provided the Oysters are not allowed 

 to become choked with the mud deposited. Too much organic 

 impurity is to be avoided. As the upper layers of water are 

 fresher than those beneath, safes and baskets used for fattening 

 purposes should be suspended near the surface. For the same 

 reason, the Oysters near the top of the rope pendants in the 

 Italian system fatten quicker than those lower down, but equality 

 can be restored by occasionally inverting the ropes. 



The lowering of the density from the 1025 of the breeding- 

 ground to 1014 at the fattening site, explains why the latter is 

 not suitable for breeding purposes, and why it is useless to set 

 collectors in the immediate vicinity. 



The fattening power of fresh water, though known to antiquity, 

 is perhaps even to-day imperfectly understood. The result is 

 certainly not due to the great nutriment contained in the fresh 

 water, for in many cases it is freer from organisms than the sea 

 water. In his * Ostricultura,' Professor Carazzi attributes the 

 effect to the influence of fresh water in increasing the action of 

 the liver, and favouring metabolism or the assimilation of food. 

 This explanation derives support from the Oyster malady, called 

 by De Montauge " hepatitis," or inflammation of the liver, 

 met with in the case of over-fed geese and birds. This 

 affection is found only in Oysters put to fatten in too fresh water, 

 and can be cured by returning the patients for a time to the sea. 

 Whatever may be the true explanation of its influence, fresh 

 water is undoubtedly desirable in a rearing and essential to a 

 fattening ground. 



