palps respond by diverting everything received, to the out-going 

 tracts. ' ' 



We thus see that everything fine that is floating in the water 

 is liable to be taken up and utilized as food by the oyster. Bnt 

 it must not be supposed that the oyster is obliged to swallow 

 everything that is brought to the mouth by the ciliary currents. 

 If the water is unusually muddy, a large amount of sediment 

 is necessarily strained out by the gills, and whenever the ac- 

 cumulation made by the gills is accessive the whole mass is 

 thrown out at one definite region. Unless there is a strong 

 current of water, this mass is not always cleared away, and. 

 since the oyster has no way of getting rid of this waste, aside 

 from discarding it from the edge of the mantle, it remains in 

 the inside of the shell in the form of a dark streak. This is 

 more or less irritating to the soft body of the oyster and so 

 it soon secretes over it a thin layer of lime or shell matter. 

 This layer is added to constantly, and eventually the streak of 

 mud may be so covered up as to be no longer visible. The 

 photograph in plate XI shows shells picked up at random from 

 oysters growing on a muddy bottom. Some of the streaks are 

 fresh and are covered by only one or two extremely thin shell 

 layers; others have lain in the shell so long, that by successive 

 layers they have become hardly noticeable. 



Generally the presence of these streaks indicates that the 

 oyster has been growing on muddy bottom, or that the water 

 contains a large amount of sediment. If the amount of mud 

 collected by the gills is so large that the oyster is continually 

 having to throw out the waste, it will partially starve itself, ow- 

 ing to the fact that both sediment and food are collected at the 

 same time, and if the former is in excess, the latter passes 

 out with it. As stated above, the oyster cannot discriminate 

 between food and that which is not, it swallowing everything 

 that is brought to the mouth by the ciliary currents of the 

 gills or so-called palps or lips. At times, and this is not in- 

 frequent on our coast, there is such an over abundance of 

 food (diatoms) in the water that this has the same effect on 

 the oyster as if it were sediment. Thus it is, that food, if collected 

 in excessive quantitives, is cast aside, and the oyster may actu- 



