v May, 1890. THE MOTION of the cilia of animalcula. 
119 
stronger ones about tlie head or mouth, the main function of 
which is to procure food or propel the creatures from place to 
place. 
Of these there are two kinds, namely, those that stand 
singly or in pairs, and those disposed in rows. The first 
kind are called flagella, and they occur mainly among the 
very small creatures, and are in consequence difficult to study 
with this apparatus. I have, however, ascertained that they 
move in a way quite unlike the others. Using a dark ground 
illumination, with direct sunlight, I was able to see that the 
flagella of Codosiga vibrate circularly, in such a way that a 
point a short distance from the free end remains still, while 
the end itself describes a small circle. The movement is 
difficult to describe, but can easily be imitated with a long 
and flexible switch. 
In the second kind of cilia, namely, those disposed in 
rows, such a motion is impossible, and accordingly we find 
them moving with a to-and-fro action. But a very little 
experience will show that it is impossible to generalise with 
regard to them. 
Some vibrate at a steady speed for hours—others give 
periodic whirls lasting half a second or more, with a pause of 
greater or less duration between each, this being especially 
the habit of the free-swimming kinds. But those in which 
the cilia are set in a circle, especially the stalked species, are 
far more regular in their habits. The Epistylis flavicans 
which I am showing to-night, and for which I am indebted 
to the kindness and good fortune of our Secretary, Mr. 
Underhill, began this evening at about ten vibrations a 
second, and is now going steadily at thirteen. 
But last night the only creature I could find in my own 
bottles was a tiny Vorticella, which was whirling away at a 
rate utterly beyond the powers of my little machine. I suc¬ 
ceeded once, by fixing the lead weights, in twirling the disc 
fast enough to make the cilia seem to stand still, and I esti¬ 
mate the speed at forty or 50 to the second. 
When we consider the small size of the creature, and the 
density of the medium with which it is surrounded, such a 
speed is truly astonishing, and must indicate an expenditure 
of force that is relatively very considerable. 
It is interesting to note the unfolding of the cilia, and 
the manner in which the action is commenced. In the 
Stentor they are raised up from the disc one after another in 
regular order, and when the last is straightened out there is 
a slight pause before the whole set start off at full speed. 
You will notice slight variations of the rate, indicated at 
