N. E. GREEN ON CILIARY ACTION IN ROTIFERA. 73 



their action reduced by this means to a fitful or languid move - 

 ment, could we arrive at a satisfactory conclusion ; but at last an 

 opportunity was afforded in the case of a very fine Stentor, who 

 found trade so bad that he had shut up shop, and seemed inclined 

 to retire from business altogether, but was induced to recommence 

 operations by the stimulant of a little fresh water. The move- 

 ments of the cilia were at first so slow and deliberate that the 

 action of each hair could be easily followed. Before the addition 

 of the water the fringe extended upright and motionless from the 

 head, but as the reviving draught passed over it, first one hair and 

 then another bent slowly towards the centre of the disc, returning 

 to the upright position at a still more leisurely pace ; then two or 

 three would follow each other gently in the same movements — soon 

 a whole side went down, but in a progressive wave, resembling 

 the action of wind on a field of corn ; and, in less time than is 

 occupied by the description, this movement became circular, in- 

 cluding the whole of the fringe, wave after wave passing round it 

 in the most beautiful and regular manner, till at last the waves 

 followed each other in such rapid succession that the eye failed to 

 follow them individually, and the illusion of the rotatory movement 

 was complete. 



On one occasion, when watching a Vorticella, the wave was ob- 

 served to rise and fall like the sullen beat of surf on the sea shore, 

 and as a wave, when falling at a slight angle with the coast, will 

 sometimes seem to dart along the shore a mile in a minute, so 

 when the movement of the cilia has been most deliberate the wave 

 has appeared to pass round the fringe like a flash. We might 

 multiply instances, but the conclusion to which we come in all, is, 

 that each cilium moves in a very elongated oval, that the greatest 

 energy of its action is inwards, that the movement is progressive, 

 one hair following very rapidly upon another, producing the ap- 

 pearance of a wave, and that these waves following each other 

 round the circle of cilia effect the illusion of rotation ; this illusion 

 being most complete where each cilium moves most slowly, viz., on 

 the outer edge. 



This peculiarity of illusion is most evident in the common 

 Kotifer, Philodina, Brachionus, and Melicerta, for in these animal- 

 cule the individual cilia are longer, and have a whip -like character, 

 the lash recurving in its stroke over the ridge on which the cilia 

 are placed. But in the Vorticella the wave-like movement is so 



