612 EXPERIMENTS WITH VIBRATING CILIA. 
most delicate efforts. There are several ways in which their 
effects may be shown without the aid of the microscope, but 
the most common is that of sprinkling some light powder over 
the ciliated membrane, from which the powder is soon swept away. 
The object of this paper is to explain some other methods adapted 
to class experiments having the same end in view, but by which 
motion is imparted to much larger masses, and also to show that 
in some instances a much greater resistance can be overcome than 
has generally been supposed possible. 
I. Experiments in water.— For these the gills of Unios and 
Anodontas are well suited. Their cilia are quite active, and vi- 
brate in such directions, that on the inner gill the motion is from 
the free edge, and on the outer to it, facts which the experimenter 
should keep in mind. If an inner gill is cut away from its at- 
tachment and laid on the bottom of a flat dish, its cilia acting as 
legs, it will soon begin to move with its free edge forwards, and will 
in the course of time, travel the entire length of the dish. We 
have seen a whole gill move ten inches in four hours. Under simi- 
lar circumstances the outer gill will move with its base or cut edge 
forwards. This difference depends, as will be readily seen, upon 
the fact that the cilia of the two gills vibrate in opposite directions. 
The result of ten experiments gaye the rate of motion of a 
_ piece of gill measuring 12mm. by 14mm., 6mm. a minute. If two 
outer gills are laid with their free edges towards each other they 
will at once begin to approach, and it frequently happens after 
meeting that one crawls directly over the other. 
Another and more striking experiment which shows the reaction 
of cilia on each other may be made as follows. Fasten a gill toa 
piece of cork under water, and place upon it a portion of a sec- 
ond gill about a half inch square. If this piece is so placed that 
the cilia vibrate in the same direction with those of the gill below, 
. it will remain stationary, or nearly so, since the cilia offer no re- 
sistance to each other. If now the upper piece is reversed 80 
that the cilia vibrate in opposite directions, the upper piece will 
move with double the speed and through twice the distance in a — 
given time that it would with its own cilia alone, for while the 
lower cilia move the upper piece through a certain space, the cilia 
of the upper piece also move this in addition through an equal 
space. A third form of this experiment consists in placing the 
upper piece so that its cilia vibrate at right angles to those of the 
