32 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 
of the same size, but two months afterwards differ 15 millimeters, 
or more than one-half an inch. 
It is not certain, of course, that the difference of rate of growth 
at different levels is always as marked as in this case, and more 
experiments must be tried to ascertain the relative growth accu- 
rately. This experiment, so far as it goes, substantiates the con- 
clusion already stated that clams grow faster when they are under 
water the greater part of the time. The reason for this is not very 
far to seek. The food of the clams consists of microscopic organ- 
isms contained in the water, which is taken in through one siphon 
and expelled through the other. The clams can feed, therefore, 
only when covered by water, and those which are located at some 
distance above the low-water mark are shut off from the food sup- 
ply during a considerable portion of each day.* 
This record of growth of the clams in the tiles is interesting 
also because it demonstrates that clams may grow during their 
first summer nearly to marketable size. The specimen in tile No. 
2 was not among the very largest of the set on the point at the 
time it was transplanted, and not larger than some of those which 
had set in the floating house-boat; so that there is no question 
about it being of this year’s production. On July 22 it measured 
15 millimeters—the size of the specimen in Fig. 20. On September 
18, in less than two months, it had more than trebled its length 
and had burrowed about six inches into the sand. The specimen 
was preserved on September 18 and is photographed natural size 
in Fig. 20 by the side of a clam of fifteen millimeters in length. 
The growth of the two adjacent specimens in the tiles Nos. 3 and 

*Since the rapidity of growth of the clams depends directly upon the supply of micro- 
scopic organisms in the water, it is to be expected that a considerably greater growth will be 
found in some portions of the Bay thanin others. I have not had an opportunity to investi- 
gate this problem carefully, but through the kindness of Mr. A. W. Bourne some samples of 
this year’s clams were obtained on December 5 which were much larger than those taken 
at the same time at Wickford. The water in the former locality is extremely rich in micro- 
scopic organsims, shown both in the skimmings taken with a tow-net and by the remark- 
able growth of animals other than clams which depend upon these organisms for food. 
