22 
precise percentage required; as it is not known at all these places how the salinity 
varies at other periods of the year. The measurements of the salinity were made 
in September 1890, but in spring the water is perhaps a little fresher, and during 
great storms also the salinity may obviously vary somewhat at many places; I 
am inclined to believe however that oysters with us can scarcely live and repro- 
duce to any considerable extent, unless the salinity as a rule and especially during 
the spawning time is up towards 3,. This is indicated by the hydrographical 
conditions at the places where the oysters are constantly found, namely, in the 
northern and eastern parts of the Kattegat, as well as in the Lim Fjord. 
To obtain information as to the degree of salinity the spat require in the 
free stage, a stage when we should expect that the salinity of the water or rather 
the specific gravity would have great influence, the spat were taken from as far 
as possible ripe oysters (thus very dark-coloured spat) and placed in several glas- 
ses containing water of a specific gravity of 1,0150,' 1,0162, 1,0173, 1,0233 all at 
a temperature of 15—189 C. After standing for a couple of days, it appeared 
that almost all the fry had sunk down to the bottom in the glass with the smallest 
specific gravity; several were obviously dead and hut very few ventured to swim 
freely around in the water. In the glasses with water of greater specific gravity, 
on the other hand, the fry continued to swim about freely for many days. We 
might therefore believe that a salinity of ca. 1,016 was sufficient for the spat in 
the free stage; but this is not definitely settled by the experiment, as I succeeded 
in none of the many cases where such fry were placed in glasses in getting them 
to fix themselves; they always died after a shorter or longer time; sometimes the 
fry lived for 14 days under these experiments. I may further state here that the 
fry can live in the free-swimming stage at a temperature of only 13? C.; this 
observation was made on board ship. In the open I have found the free-swimming 
oyster spat in water of only 15? C. At a temperature of ca. 19? C. or less they 
can thrive and fix themselves; thus in the first days of August 1895 the quite 
small spat (2— 13 mm.) were found on the collectors set out by me and on other 
objects present in the water. The temperature was taken each morning about 7 
a. m., and during the whole of the month of July 1895 the highest temperature 
observed was 19,4" C. the lowest 15? C. During the same period the specific 
gravity in Nykjøbing Harbour, where the oyster spat was also found in July, 
varied between 1,02000 and 1,0239. 
Temperature curves for Oddesund 1893—95 and for Nykjøbing 1895 are 
appended in order to give some notion of the temperature in the summer half-year. 
Ripe spermatozoa with distinct tail were found for the first time in an 
oyster at Nykjøbing on May 24th 1895; but few were investigated. On June 14th 
20 oysters from Ørodde were examined; 3 of them had white spat inside their 
shells, one had blue spat. 
On June 1åth 13 oysters over 2 inches were examined from Sallingsund, 
b of which had blue and 2 white spat. The spat was now obtained daily in our 
plankton net both in the harbour and in Sallingsund; it was present however in 
much smaller numbers than the larvæ of other molluscs. Both night and day, 
in bright, sunny and in dull weather the spat was found on the surface, but 
constantly also in deeper wuter; it seems (lo be present everywhere and at all 
