On (Ectstes umhella and other Rotifers. By C. T. Hudson. 7 



the " chases " are ; but I am inclined to think that the effects 

 witnessed are rather due to the friction between the sides of the 

 " hopper " and the stream itself. Anyone who has sat in a boat 

 floating down a swift stream must have noticed that light floating 

 particles on the surface pass him, that they are going at a quicker 

 rate than his boat is ; and that anything like a free buoy, which in 

 still water would float upright, is in the swift stream tilted fortvard 

 as it floats, its submerged end as it w^ere dragging behind the free 

 top. All this is clearly due to the fact that the upper portions of 

 the river are flowing faster than the lower, which are hindered by 

 the friction of the water against the channel itself. In the same 

 way the side portions of the stream close to the banks move per- 

 ceptibly slower than those farther off", and very much slower than 

 the centre of the stream. The result is that while the heavier 

 floating bodies, owing to their greater momentum, generally escape 

 from the feebler currents if they ever get into them, the very light 

 particles (often pushed aside and towards the banks by the heavier 

 ones) are constantly caught and retained by the gentle currents at 

 the side. I think then that the minute particles pass slowly along 

 the " chases," merely because along the chases run comparatively 

 feeble currents, owing to the retarding action of the sides of the 

 " hopper," and especially of its curved edges. 



1 should be inclined to think also that the production of the 

 peculiar form of the pellet is due to mechanical considerations out 

 of Melicerta's control. For instance, the pellet is frequently seen 

 to rotate in one direction round its axis, and then after a few 

 revolutions to rotate in the opposite direction round the same axis ; 

 and to repeat this again and again with great regularity, the 

 coloured specks on the pellet even enabling the observer to time 

 the process. Now at first sight this looks as if Melicerta had 

 reversed the action of its cilia in the cup at its own pleasure ; but I 

 believe that there is a simpler explanation. The cilia wdth which 

 the cup is lined, suddenly curving inwards in turn one after another 

 — just as on the trochal disk — produce a vortex in one constant 

 direction so long as the pellet is small enough to lie clear of all 

 of them, but when it gets larger it hinders the action now of one 

 portion of the cilia lining the pellet, now of another, by getting 

 so close to them as to stop their blows, and then the cilia on the 

 opposite side to the checked ones have the advantage and produce 

 a current towards themselves, which not only makes the pellet 

 rotate round its axis from the checked ones toivards themselves, 

 but at last draws it bodily over to the side where the cilia are free, 

 thus checking in their turn those previously free and releasing 

 those previously checked. Of course, the rotation is at once re- 

 versed till the pellet is drawn back to its old position, and then 

 da cai)o. That the pellet is not truly spherical is, I think, mainly 



