MICROSCOPIC AQUATIC ORGANISMS. 361 



general occurrence even among these. The best-known group 

 in which they are found is the Flagellata or Mastigophora, a 

 subdivision of the Infusoria. They also occur in many different 

 kinds of Algae and Bacteria. 



Leaving the subject of locomotion due to flagella, it is an easy 

 passage to that caused by the action of cilia. Here, again, a 

 definition is not altogether free from difficulty ; but my own 

 conception of a cilium is that it is, like a flagellum, a permanent, 

 motile, whip-like extension of the living cell-substance, but that, 

 unlike a flagellum, it is incapable of moving the body to which it 

 is attached by its own individual efforts (being usually small in 

 comparison with the body), while it is always associated with a 

 number of other cilia with which it acts in harmony. Probably 

 the mode of its action is also characteristic. 



A large amount of attention has been given to the structure of 

 cilia, and it has been maintained that they consist of a number 

 of separate parts, namely, root (sometimes connected with the 

 nucleus), im motile rod, basal granule, bulb, and terminal vibra- 

 ting piece. This complicated structure, however, is certainly not 

 always present. In fact, there are excellent reasons for thinking 

 that the only essential part of a cilium is the vibrating thread. 

 Of course, the connection with the nucleus, when it occurs, seems 

 to show that the control of the cilium may be from within the 

 cell, but the motive power is pretty certainly contained within 

 the cilium itself. Cilia may occur almost uniformly covering 

 the whole body of an organism (many Infusoria, Planarians, etc.), 

 or they may be limited to special areas or arranged in bands, 

 wreaths, and other ways (Infusoria, Rotifers, Gastrotricha, etc.). 

 Various theories have been put forward to account for the 

 locomotion produced by cilia. The simplest of these, and the one 

 generally accepted, is that the cilia act with a simple lashing 

 movement in one plane, but with the motion in one direction 

 much quicker, and therefore stronger, than in the other. Such 

 action has been directly observed in some cases, and the suppo- 

 sition that it is the normal movement of cilia fits in very well 

 with the observed facts of the locomotion of ciliated organisms. 

 Usually the successive rows of cilia exhibit a slight retardation 

 in the moment of lashing, so that an appearance is produced as of 

 waves travelling over a field of corn. Sometimes the illusion 

 produced by these waves is so marked that it seems to be an 



