iNFusorviA. 67 



spiral. In Trichodina there is a crown of cilia on 

 the back, in addition to which a peculiar undula- 

 tory merabrane, richly furnished with these minute 

 organs, occurs on the ventral surface of the body. 



In form the cilia may be described as elongated, 

 broader at the base than at the tip, being usually 

 somewhat flattened. They vaiy in length from 

 •02 to about '00005 of an inch. Their motion is 

 mostly uniform, each of the cilia bending in rapid 

 succession from its base to its point, and returning 

 immediately to its original condition : sometimes 

 these movements suddenly cease, but after a mo- 

 ment's pause they are again resumed either in 

 the same or in an opposite direction. From their 

 minute size the cilia are often difficult of detection. 

 Their presence, in many cases, can hardly be ascer- 

 tained until their motion has very much slackened, 

 or it may be indirectly inferred from the agita- 

 tion of floating particles caused by the cm-rents 

 w^hich are excited in the surrounding water. 



By means of these cilia, the Infusoria move ra- 

 pidly about in the water wherein they are found, 

 and it is curious to observe how, when a number 

 are confined to a small portion of that fluid, they 

 rarely seem to come into collision with one an- 

 other or any obstacles which may be placed in their 

 way. 



Besides the true cilia, other appendages, of ap- 

 parently similar natm-e, but larger size, are met 

 with among many Infusmia, Such, for example, 

 are the ^ setae' or ciliary bristles of Oxytricha 

 (/5''T3, c), the ^ uncini ' (hooks) and 'styles' of 

 Euplotes (e, e'), and the 'flagelliform filaments' 

 of Peranema (/). The latter may be described as 

 long filamentous prolongations, proceeding from 



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