146 



from the shore, and had at this time the exact temperature of the 

 open lake, the conditions for experiment were as favorable as arti- 

 ficial arrangements could well be made. 



Sending a man with a towmg net out upon the lake with a boat, 

 or upon the remotest In-eakwaters, immense numbers of all organic 

 objects in the water were easily obtained. After enclosing the exit 

 of the tank with a fine wire screen, to prevent the escape of 

 objects placed in it, we poured these collections of all descrip- 

 tions indiscriminately into the water^ from day to day, thus keep- 

 ing the fishes profusely supplied tvith all the various kinds of 

 food which could possibly be accessible to them in their native 

 haunts. From this tank one hundred fishes were taken daily and 

 placed in alcohol for dissection and microscopic study, to determine 

 precisely the objects preferied by them for food. These were ex- 

 amined at a later date, and all contents of the intestines were 

 mounted entire as miscroscopic slides, and permanently preserved. 

 A careful study, was of course made of the organisms of the lake, 

 as shown by the product of the towing net, and when the experi- 

 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 present, and 

 the median fin extended from the tips of the pectorals on the belly 

 to a pomt opjDosite 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 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 temperature, 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 insuring 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 Algip. and 

 many Entomostraca, the latter belonging chiefiy to the genera 

 Cyclops, Diaptomus, and Limnocalanus among the Copepoda, and 

 to Daphnia among the Cladocera. 



As th: 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 



