FOOD AND FEEDING OF OYSTERS. I307 



The oysters prepared in accordance with the foregoing description are then 

 placed for about three days in filtered sea water, renewed morning and night, 

 at the end of which time they are practically purged of food and usually gaping 

 with hunger. The intestinal contents of five are then determined by means of 

 the apparatus and methods already described. 



The remaining oysters are now placed each in a 6-inch Petri dish, the shells 

 resting on a wire support to raise them above the bottom and lying so that the 

 deep valve is downward and in such position that the cloacal or excurrent cham- 

 ber of the oyster lies below the apron and the oral or incurrent chamber above 

 it. The " apron " is then confined to the sides of the dish by means of a rubber 

 band or cord, and a layer of sand is placed over the window and the surround- 

 ing rubber to serve as a filter, as shown in figure 4. A piece of cheese cloth tied 

 over the sand will prevent its being washed away by currents or disturbed by 

 inquisitive fishes and crabs. 



When the oysters thus prepared are transferred to their natural environment 

 they are as free to open their valves and feed as if they had never been removed 

 from their beds ; the oral chamber is in unobstructed communication with out- 

 side waters while the excurrent chamber discharges into the Petri dish, where the 

 faeces are retained while the expelled water passes through the filter. Oysters 

 prepared as described have been kept under close observation under otherwise 

 natural conditions and appeared to feed as freely and normally as neighboring 

 specimens that had never been disturbed. The faeces drop into the dish in a 

 little heap of demicylinders, while extraneous matter was excluded by the apron 

 and filter. 



At the end of three and six days, respectively, lots of five of these oysters are 

 taken up, their intestinal contents removed by the method already described and 

 added to the faeces collected from the dishes. As about 95 per cent of the food 

 consists of diatoms whose tests pass unchanged through the alimentary canal, 

 it is evident that by calculating the volume of the combined food organisms of the 

 faeces and alimentary canals by the methods described and deducting the 

 volume of the residual intestinal contents, as determined from the lot of five 

 starved check oysters, we can arrive at a volumetric expression of the average 

 rate of feeding. Determinations of the diatomaceous content of the surrounding 

 water made at intervals during the experiment supply the data for the necessary 

 correction to be applied for dead diatom frustules ingested by the oysters under 

 experiment. 



It is perhaps not necessary to use starved oysters for these experiments, but 

 they have been used in order to insure the prompt commencement of feeding as 

 soon as returned to the water, unstarved specimens sometimes "sulking" 

 after repeated handling. A check upon possible error due to any abnormal 



