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 Algii?, 

 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 tlie 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 Cijclops 

 thomasi.* This little Entomostracan is only .04 inch long, by .011 



*0n some Entomostraca of Lake Michigan and Adjacent Waters. 

 American Naturalist, Vol. XVI., No. VIII, August, 1S82, pp. 640 and 

 649. 



