76 DEPARTMENT OF THE NAVAL SERVICE 
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7 GEORGE V, A. 1917 
‘made a rim of 7% mu around its edge. As most of the larve are 400 mu high, from. 
tip of left umbo to edge of right valve, it follows that spat growth can best be 
indicated by omitting this “constant” from the total measurement, which will hence- 
forth be done. 
August 26, the experimental shells which were placed on the 12th and the 16th, 
were taken for examination. No spat were found on the shells placed August 16, but 
a third of the shells placed August 12, carry spat up to a diameter of one millimeter. 
As no spat were found on the shells placed on the 16th, the inference would be that the. 
spatting occurred before the 16th, which, taken in conjunction with the fact that these 
spat were of nearly the same size as those seen August 24, on shells planted for a week, 
leads us to the eonelusion that in both cases we have to do with the setting of spat that 
showed as “ ready to set” in the plankton of August 14. It might, however, not be true 
that the shells placed August 16 failed to catch spat, because all had set that were ready. 
Possibly none were in the water at that point, and this supposition becomes probable 
when we study the shells taken from the McNutt bed, next to be considered. 
Assuming the 14th as the probable date of first spatting, we get the tentative result 
of about 100 mu growth of spat shell per day. 
On the 26th we “lifted” several oysters and shells from the McNutt beds, and 
these showed spat very much like those in the McNeill samples. The most spat were 
found on the inside of oysters that had died and decomposed recently, leaving clean 
inside surfaces, well protected from entrance of both silt and the larger enemies, such 
as snails, because the valves of the oyster shell naturally separate only narrowly. A 
study of the distribution of these spat is instructive. ‘The number of spat on the out- 
side was equal for both valves, but totalled only one-eighth of the number found inside. 
There were twice as many inside spat on the right valve as on the left. or lowermost 
valve, even in the instance where both valves were absolutely clean. The number was 
in all cases proportional to the cleanness of the surfaces, ranging for the inside upper 
valve from, 1 to 150 spat per shell. The highest number was on a small sheil, and the 
spat were most beautiful, showing what nature can do even with limited resources, if 
given a fair chance. We should also note that the spat prefer to set on the under side of 
an object, even when the surface is no cleaner or otherwise better than in other positions. 
The European oyster farmer takes advantage of the fact in his method of tile culture. 
In short, the spat like a “roof over foot.” This is the result of natural selection, as 
those fry that possess the instinct to set under a surface, are not so apt to be smothered 
by silt, and also they find less silt to scrape away to get a hold. 
The spat shells were measured in nearly fifty instances on the best set cultch 
sample and we found all stages present, from spat newly set, up to those having 1200 
mu of spat-shell. Sizes 150, 400, and 600 mu had the most numerous representation. 
Allowing 100 mu growth per day, we get twelve days as the age of the oldest, which 
brings the date of beginning of spatting to be the 14th, quite in harmony with the 
plankton evidence. The main spatting period was from August 20 to the 22nd. This 
is in harmony with the figures in the plankton table for this period, showing few fry 
in stage V, because they were exploring the bottom at the time. As the climax of the 
spatting occurred on the 20th, and no spat were found on the shells placed on the 16th 
(taken on the 26th), it is evident that no fry ready to set were present at that locality. 
Still farther from the wharf were the Reilley experimental shells; they were placed on 
the 23rd and taken up on the 27th, and no spat were present on them. So here, too, 
was an area which was poor in spat, at those dates at least. Just how far fry may 
wander from their birthplace, during the weeks of their plankton life, is not known, 
but it is a possibility that they do not wander far. This is a subject of great import- 
ance, and deserves careful research. While they are in the plankton condition they 
are a part of the water, and they use their swimming powers to rise or to sink. By 
rising into the tide early in flow, and settling to the bottom before ebb begins, it is 
evident they can wander as far from home as the distance travelled by a tide in six or 
seven hours. This would not distribute them laterally, to the current, except when 
