ORGANS OF LOCOMOTION. 29 



tion, and, indeed, it is one of the most interesting 

 spectacles that can be observed by the aid of the 

 microscope. It is supposed by some naturalists 

 to be also the means of respiration. In some 

 genera the cilia are not confined to the opening, 

 but cover the whole body, and by this means 

 they progress rapidly through the water, having 

 a revolving motion, like that of a ball. Some 

 have a gliding movement, similar to the snail 

 or leech ; others, again, have a foot, with which 

 they catch hold of substances that come in 

 their way. In many species, also, bristles 

 are attached to the surface of the body, 

 which may act as fins ; and some kinds have 

 appendages, curved at their extremities, and 

 serving as hooks to attach the animal to sub- 



D 3 



