FOOD OF THE OYSTER, CLAM, AND RIBBED MUSSEL. 379 



Average samiile of the excrements of 4 clams, collected on the Jlats at the Soldiers' Home, 



Haynpton, Va. 



Results. 



Results of Preparation No. 1 : 



Per ct. 



Living 2 



Entirely dij;;ested 83 



Nearly digested 15 



Total 100 



Results of Preparation No. 2 ; 



Living 



Entirely digested 86 



Nearly digested 14 



Total 100 



Average : 



Digested 99 



Entirely 84. 5 



Nearly 14.5 



Living 1 



Total 100 



* Probably derived from the sea water used to break up the excrements. 



There was another possible kind of food yet to be considered, 

 namely, such substances as might be in solution in the water in which 

 the oyster lives. The oysters attached to the stones of the Eiprai)S, 

 which also showed their stomachs full of diatoms, are surrounded by 

 perfectly clear water, but from a large number of chemical analyses 

 which I have made of similar water on former occasions, after having 

 strained the diatoms, etc., out, I am convinced that hardly a trace of 

 organic matter is to be found dissolved in it, so tha^" this possible source 

 of food can be entirely excluded. 



After having determined in this way that the food of the oysters 

 and clams in the James Eiver consists i^ractically of diatoms, the 

 question presented itself. Where do these diatoms come from? The 

 use of the common Miiller's pelagic tow net revealed their presence at 

 the surface of the water in enormous quantities, and no difference 

 could be detected in their numbers or distribution during the daytime 

 or nighttime. The occurrence of diatoms in such numbers at the 

 surface explained well their presence in the stomachs of the oysters 

 attached to the stones and piles submerged only a little under water, 

 but this could not account for their presence in those living at the 

 bottom in deeper places, where even at low tide considerable water 

 remained. It was therefore thought advisable to collect at different 

 depths in order to study their i^erpendicular distribution from the 

 surface to the bottom. The result showed that they occurred in equal 

 quantity at all depths up to 70 feet, which, according to the official 

 max)S, is the greatest depth found at the mouth of the James liiver. 



