STUDYING THE HABITS OF FISHES. II27 



The nest-building process. — Presently the fish will be seen to put its head 

 to the bottom of the pit and by vigorous movements of its tail appears to try 

 to force its way into the bottom, as in figure 1 1, plate cxix, in which it is seen to 

 have seized a pebble. Presently it rights itself and is then seen to have picked 

 up a good-sized stone from the bottom of the pit (fig. 12, pi. cxix). With this it 

 swims to the lower end of the ridge of gravel and there drops it (fig. 13, pi. cxix), 

 so that it falls on the end of the ridge or rolls down into the pit. In either case 

 it helps to lengthen the ridge. Sometimes a stone too large to be taken into the 

 mouth is pushed along the bottom (fig. 14, pi. cxix). The fish now carries stone 

 after stone in this way until the ridge is visibly lengthened. Sometimes instead 

 of a single larger pebble the fish takes into its mouth a mass of smaller pebbles, 

 with a considerable amount of sand. When this happens, he does not drop 

 his burden on the end of the ridge, so as to lengthen it, but proceeds some 

 little distance farther upstream, until his head at least is well above the 

 ridge. Then with a movement of his head first to one side and then to the 

 other he distributes the mouthful of small pebbles over the top of the ridge, 

 so as to form a top dressing. As the pebbles leave his mouth it may be seen 

 that the sand which was taken up with them is washed downstream and falls 

 to the bottom below the nest, where it forms the trail already seen (fig. 4, text, 

 at right) . This trail is added to by the sand stirred from the bottom of the pit 

 by the fish whenever he picks up a stone. As the ridge lengthens it slowly 

 encroaches on the pit and tends to fill it. But as this occurs at the upper end of 

 the pit the fish slowly pushes his excavation downstream, lengthening the pit at 

 the lower end, so that it does not become filled. The whole ridge thus lies in 

 a long trench, which has been excavated as the fish slowly drops downstream 

 and has been filled by the ridge as fast as made. Only the pit in which the 

 fish lies has been left unfilled at the lower end of the ridge. If the observer is 

 fortunate enough to arrive as the dace is beginning its nest, he may see it dig 

 a little pit in the level bottom and pile the stones on its upstream edge (fig. 13, 

 pi. cxix). By gradually lengthening this pit and at the same time filling it 

 he completes his nest. 



The structure of the completed nest is shown in perspective in figure 9, 

 plate ex VIII, and as it appears in longitudinal section in the diagrammatic 

 figure 3, page 11 26. At the right of text figure 4, page 11 28, it appears in plan. 

 The bottom of the stream is here composed of gravel, with sand intervening 

 between the stones. In this the long trench has been excavated and partly 

 filled with the stones that have been removed in digging it. These stones may 

 be distinguished from those that still remain undisturbed in the bottom by the 

 fact that they are clean and the sand has been washed from between them. 

 They form a low ridge, which projects from the trench somewhat above the 



