706 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [24] 
to only one of them, until finally but a trace of its fibers is to be seen at 
the points of attachment, the so-called muscular-impressions. During 
the latter part of March, 1870, I was able to follow out for myself, at the 
Schleswig-Holstein beds, the entire course of the changes produced in the 
oyster by freezing. Long-continued east winds had kept the water ex- 
traordinarily low, and for more than a month thick ice had covered the 
flats, so that from the 4th of February to the 7th of March no oysters 
could be taken. On the 14th of February the water in the neighbor- 
hood of an oyster-bed at the north end of the island of Sylt was found 
to be of a temperature of 2° C. below zero. At this point the depth 
of water was 3.5 meters. Of those oysters which were taken in my 
presence from the shallower beds 7 to 8 per cent. were frozen. Upon 
beds which lay in deeper water, nearer the open North Sea, the cold had 
killed only from 2 to 3 per cent. Evidently, then, these latter beds had 
suffered less damage because at every flood-tide they received water of 
a somewhat higher temperature from the open sea. I have frozen the 
mantle and gill lobes of oysters in North Sea water and allowed them 
to remain inclosed in ice for an hour at a time, with the temperature of 
the water varying in degree from 4° C. to 9° C. below zero. When the 
ice had melted, the cilia began to move feebly, and four hours later, 
when the temperature of the water had risen to 5° C. above zero, their 
movements were once more fully established. Other gill and mantle 
lobes which had been three hours in water of a temperature of 1° C. to 
2° ©. below zero moved quite lively on the following day. This 
recalls to me a very weighty difference between fresh and salt water, 
which is often overlooked. It is generally known that fresh water 
is densest and heaviest at a temperature of 4° C. above zero. When any 
portion has arrived at this temperature during freezing weather, it sinks 
to the bottom of the body of fresh water, where it remains until the en- 
tire mass above it is of the same density. That portion which first be- 
comes lower in temperature than 4° C. then expands, rises to the surface, 
and stiffens into ice as it reaches the temperature of 0°. 
The fact is less known that with sea-water the lower the temperature 
the greater the density and weight of the water. Therefore, it also sinks 
to the bottom until it has reached the temperature at which it forms ice, 
which, when it holds 3 per cent. of salt in solution, is 2.28° C. below 
zero. It is evident, then, that water may be found at the bottom over 
the sea-flats of a temperature of 2° C. below zero, while, during the most 
severe cold, water at the bottom of the lakes and deeper rivers of North 
Germany is found to be constantly several degrees warmer than this, 
When, finally, the sea-water, from the surface to the bottom, has reached 
its freezing point, it does not become solid ice for the whole thickness, 
but thin layers of ice, at greater or less distances apart, are formed in it. 
These layers, which are crystallized from the salt water, are free from 
salt, are hence lighter than the surrounding water, and accordingly as- 
cend to the surface; consequently, those animals which live upon the 
