Our Food Mollusks 



over oyster beds. The floats are about 

 eight inches long, and nothing else is 

 required but a vessel that will hold a 

 column of water of that height, in 

 which the apparatus may be immersed. 

 Temperature corrections are not neces- 

 sary in ordinary observations. 



Surface and bottom densities often 

 vary greatly, and obviously that at the 

 bottom is the one of greatest interest 

 to the oysterman. It may be obtained 

 by sinking a corked vessel and then re- 

 moving the cork by means of a cord. 



Oysters are brackish water forms. 

 Their natural distribution has always 

 been close to the shore where waters 

 are fresher than in the open sea. If 

 other conditions are favorable, they 

 will exist in very salt water, but grow 

 little, and do not reproduce. On the 

 other hand, mature oysters have been 

 known to live for some time in water 

 nearly fresh. Such an experience, 

 however, is always harmful. It some- 

 times happens, as in the Carolina field 

 and in the Gulf of Mexico, that long 

 continued freshets cause widespread 

 destruction. On a considerable part 

 of the latter shore, especially, the 

 freshening of the water is a con- 

 tingency on which the oyster culturist 

 must take his chances. 

 The extremes of salinity between which the growth of 



FIG. 24. Salino- 

 meter. A glass 

 bulb weighted 

 with shot and 

 carrying a scale 

 for measuring 

 the amount of 

 salt in sea- 

 water. 



