The First Food of the Common White-Fish. 105 
ment was finally ended, an equally careful examination followed 
of the living contents of the water of the tank at that time. 
These fishes, like those previously described, had already 
reached the age and condition at which it is customary to “ plant” 
them in the lake. The ventrals were still undeveloped, the egg- 
sac had nearly disappeared, the four mandibular teeth were pres- 
ent, and the median fin extended from the tips of the pectorals on 
the belly to a point opposite the middle of the same fins on the back. 
In most the egg-sac did not protrude externally, being reduced in 
some to a droplet of oil, but remaining in a few of a size at least 
as great as that of the head. The alimentary canal was of course 
a simple straight tube, without any distinction of stomach and 
intestine. 
The sufferings of these fry in transit had doubtless weakened the 
vitality of the survivors, and although every care was taken to 
keep the water of the tank fresh and pure, about one-third of 
those remaining died during the progress of the experiment. The 
aquarium in which they were confined was built of glass, and 
had a capacity of about one hundred cubic feet. The tempera- 
ture, tried repeatedly, stood at forty-two degrees Fah. A steady 
current of the water of the lake was maintained through this tank, 
entering through a rose, from which it fell in a spray, thus insur- 
ing perfect aeration. 
By far the greater part of the organic contents of the water of 
the lake, as shown by the product of the towing net, consisted of 
diatoms in immense variety, which formed always a greenish* mu- 
cilaginous coating upon the interior of the muslin net. In this 
were entangled, a variety of rotifers, occasional filamentous Algie, 
and many Entomostraca, the latter belonging chiefly to the genera 
Cyclops, Diaptomus, and Limnocalanus among the Copepoda, and 
to Daphnia among the Cladocera. 
As the Entomostraca proved to be far the most important ele- 
ments of this food supply, the particulars respecting them may be 
properly more fully given. The smallest of all was a Cyclops, 
then new, but since described by me under the name of Cyclops 
thomcisi .* This little Entomostracan is only .04 inch long, by .011 
*On some Entomostraca of Lake Michigan and Adjacent Waters. 
American Naturalist, Yol. XVI., No. VIII, August, 1882, pp. 640 and 
649 . 
