Methods Employed. 2(55 



wandered more than a short distance from the dock and always returned 

 to it. I do not of course know that no changes took place in the colonies 

 at night, but the results of the experiments as detailed below indicate that 

 such changes must have been few. 



The gray snapper feeds in the daytime and by sight. It usually ap- 

 proaches its prey slowly from below. When near enough (i to 3 feet) it 

 strikes quickly and, judging its distance with great accuracy, seizes the food, 

 turns sharply, and returns to near its starting-point. The whole movement 

 greatly resembles that of the end of a whip-lash when the whip is cracked; 

 hence probably the name " snapper." The fish were entirely at liberty during 

 the experiments and were taking their normal food and leading their normal 

 life in an unmodified environment. The Laboratory colony was habituated 

 to the frequent presence of people on the dock and paid little heed to them 

 so long as they did not approach nearer than about 15 feet. This colony 

 occasionally received food thrown from the dock and the fish were then 

 accustomed to assemble, but the food received in this way was not enough 

 to affect their normal appetite. They were always hungry. This feeding 

 was, moreover, discontinued during the period covered by the more im- 

 portant experiments. That it did not aflfect the experiments appears from 

 the fact that the fish at the other docks were not fed, and yet their behavior 

 was identical. 



The commonest food of the gray snapper in June and early July is the 

 so-called sardine or hard-head {Atherina latieeps), a silvery-white fish about 

 2 inches long. It occurs in immense schools along the shore and is con- 

 stantly pursued by gray snappers. To test the power of the gray snappers 

 to discriminate colors, from associations, and retain them, atherinas were 

 dyed various colors, as described below, and thrown from the dock to the 

 snappers. The experiments were therefore mass experiments, in which 

 an entire colony of gray snappers participated without being removed from 

 theii accustomed habitat. 



The water is very clear at the Tortugas, and it is usually calm on one side 

 or the other of the fsland, so that the observer on the dock sees the snappers 

 clearly and is able to record the results of an experiment without difficulty. 



