FOOD OF THE OYSTER, CLAM, AND RIBBED MUSSEL. 377 



stomachs; and, second, what part of the additional decaying organic 

 matter was digested. It was possible, for example, that the diatoms, if 

 abundant in the surrounding water, were merely ingested and would 

 pass the intestinal canal unchanged, while the decaying organic matter 

 might be digested. In order to settle this point, several oysters and 

 clams were placed in separate glass dishes, their shells being previously 

 carefully cleaned with a brush. The sea water in these dishes was 

 either naturally very pure or strained through filter paper; after a few 

 hours a considerable quantity of ffeces was deposited in the dishes. 

 The excrements of the oyster, as well as those of the mussel and soft 

 clam, are well formed, consisting of a hollow tube or of a solid rod of 

 excrementitious substance; the amount of sand in them is enormous, 

 forming by far the greatest bulk. 



The cell walls of the diatoms, on account of the silica which they 

 contain, are indigestible; for this reason it was easy to determine with 

 accuracy whether digestion of the diatoms actually look place, as it 

 had been previously ascertained that very few empty shells of diatoms 

 were present in the stomach, by far the greater number being in fresh 

 condition. The examination of the excrements under the microscope 

 showed that the decaying organic matter had passed through the ali- 

 mentary canal entirely unchanged. At first it seemed as if the diatoms 

 also were very imperfectly digested, but soon it became evident that 

 this was an error based on superficial examination, since the undigested 

 diatoms were more conspicuous on account of their coloring matter, 

 while the delicate transparent shells of those which had been digested 

 escaped observation. To avoid this error the following method was 

 adopted : The excrements of a certain number of oysters or clams were 

 collected, broken up in water, and well mixed. From this average 

 sample two preparations were made and in each of these twenty-five 

 fields selected at random were counted. I have tabulated below the 

 results of the examination of two such samples. The great difference 

 in the number of diatoms present in each field is due to the fiict that 

 the samples were very differently diluted with water. 



In Column I, under "dead," is recorded the total number of dead 

 diatoms observed. The letter d following a number indicates that not 

 all the diatoms were completely digested, although by far the most 

 were nearly so, only a little of the coloring matter remaining. 



Column II shows the number not entirely digested, and the difference 

 between the numbers in the two columns indicates in each instance the 

 number in which only the clean silica skeleton remained. 



In none of these cases was the additional decaying organic matter 

 digested. The numbers of individuals examined being very different, 

 having been taken from different localities and representing different 

 genera, the fact that the results coincided so closely in the proportion- 

 ate number of digested and undigested diatoms seems to indicate a 

 very complete digestion of the ingested diatoms in this group. 



