Li 
FOOD OF THE OYSTER, CLAM, AND RIBBED MUSSEL. 377 
‘stomachs; and, second, what part of the additional decaying organic 
imatter 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 
‘earefully 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 feeces 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 took 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 fact 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. 
