24 DISCOVERT 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 withdrawn into the substance of the body, and the whole organism, hitherto ellip- 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. 



