24 DISCOVERY OF THE CELL. . 
one pole of a lighter green than the other; it moves always in the direction of the 
former, so that the lighter end may be properly designated the anterior. At first 
the ball rises to the surface of the water towards the light, but soon after it again 
sinks deep down, often turning suddenly half-way round and pursues for a time a 
horizontal course. In all these movements it avoids coming into collision with the 
stationary objects which lie in its path, and also carefully eludes all the creatures 
swimming about in the same water with it. The motion is effected by short pro- 
cesses like lashes or “cilia,” which protrude all round from the enveloping pellicle 
of the jelly-like body and are in active vibration. With the help of these cilia, 
which occasion by their action little eddies in the water, the whole ball of green 
jelly moves in any given direction with considerable rapidity. But at the same 
time as it pushes forward, the ellipsoid turns on its longer axis, so that the resultant 
motion is obviously that of a screw. It is worthy of note that this rotation is 
invariably from east to west, that is, in the direction opposed to that of the earth. 
The rate of progress is always about the same: a layer of water of not quite two 
centimetres (1°76 cm.) is traversed in one minute. Now and then, it is true, the 
swimming ellipsoid allows itself a short rest; but it begins again almost immediately, 
rising and sinking, and resumes its movements of rotation and vibration. Two hours 
after its escape the movements become perceptibly feebler, and the pauses, during 
which there is only rotation and no forward motion of the body, become both longer 
and more frequent. 
At length the swimmer attains permanent rest. He lands on some place or 
other, preferably on the shady side of any object that may be floating or stationary 
in the water. The axial rotation ceases, the cilia stop their lashing motion and are 
withdrawn into the substance of the body, and the whole organism, hitherto ellip- 
soidal and lighter at its anterior end, becomes spherical and of a uniform dark- 
green colour. So long as it is in motion the gelatinous body has no definite wall. 
Its outermost layer is, no doubt, denser than the rest; but no distinct boundary is 
to be recognized, and we cannot properly speak of a special enveloping coat. No 
sooner, however, is the ball stranded, no sooner has its movement ceased and its 
shape become spherical, than a substance is secreted at its periphery; and this 
substance, even at the moment of secretion, takes the form of a firm, colourless, and 
transparent membrane. Twenty-six hours afterwards, very short branched tubes 
begin to push out from the interior, and these become organs of attachment. In 
the opposite direction the cell stretches into a long tube which divides into branches 
and floats on the water. After fourteen days the free ends of this tube and of its 
branches swell once more and become club-shaped; a portion of their slimy contents 
is, as before, separated from the rest and liberated as a motile body, and the whole 
performance described above is repeated. 
