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BULLETIN OF THE UNITED STATES FISH COMMISSION. 
accuracy, so far as I could see, in that they at once swam away from such centers 
of disturbance as come from dropping a stone in the water. In this last particular 
they were very unlike the fishes reported on by Bonnier (1896, p. 918). In one 
important point they differed absolutely from the normal fishes; they would swim 
upward and remain near the top during even a considerable agitation of the whole 
aquarium, though they would dart downward at any sudden movement on the part 
of the observer. Hence these fishes must have lost their capacity to be stimulated 
by the mass movement of the water, and since this defect was observed only after 
the lateral-line organs had been rendered inoperative, I concluded that the normal 
stimulus for these organs was a very slight mass movement of the surrounding 
water. Since such movements always accompanied the sound produced by the bass- 
viol string, it follows that the disturbances set up by this string must have acted as 
a stimulus for the lateral-line organs as well as for the ears, and it is therefore not 
surprising that earless fishes sometimes reacted when the string was plucked. 
If the lateral-line organs are stimulated by a slight mass movement of the water, 
it occurred to me that I ought to be able to separate, in a mixed school of fishes, those 
with lateral-line organs intact from those in which the nerves to the organs had 
been cut, by simply imparting a slight mass movement to the water. Under such 
conditions the normal fishes ought to swim to the bottom, leaving the defective ones 
above; but on trying the experiment I found that the fishes were so accustomed 
to form a school that when the normal ones started for the bottom the others did 
the same, and I was entirely unable by this means to separate the normal from 
the defective individuals. But I finally succeeded in doing this by modifying the 
experiment, in that I used only two individuals, one normal and one defective, and 
agitated the aquarium only when they were widely separated. The result was very 
decisive in that the normal one invariably took the initiative in descending, and in 
fact was often not followed by its defective companion. 
Having found the conditions under which the lateral-line organs were stimu- 
lated, it is natural to inquire as to the exact nature of the stimulus. Ordinarily the 
fishes were induced to react by making the whole aquarium swing at about ten 
vibrations per second ; but a like reaction was obtained from the normal fishes when 
a single swing, or what was as near as possible a single swing, was given to the 
aquarium. The stimulus therefore is not necessarily of a vibratory kind, but con- 
sists in a slight movement of the body of water as a whole. It might be supposed 
that since the fish was suspended in the water, the motion of the aquarium as a 
whole could have no influence on it. But it must be remembered that the fish was 
somewhat heavier than the water, and that each time the aquarium was moved the 
fish, from its inertia, must have lagged a little behind or, once set in motion, moved 
a little ahead, and it is this slight difference in the rate of movement of the fish and 
of the adjacent water that, in my opinion, induces stimulation. I am not prepared 
to say how this affects the sense organs in the lateral-line canals; but it is not 
impossible, as Schulze (1870, p. 85) suggested, that slight currents are thereby set 
up that move and thus stimulate the bristle cells of the lateral-line organs. 
The extreme sensitiveness of animals to slight motions of this kind has already 
been pointed out by Whitman (1899, pp. 287 and 302) in the leech and salamander, 
and I suspect that the sensitiveness of the blind fish, as observed by Eigenmann 
and quoted by Whitman (1899, p. 303), may also be in the nature of a lateral-line 
response. 
