140 THE MECHANICAL FUNCTIONS. 



through the fluid, produced apparently by the vibrations of 

 cilia, which are set on various parts of the body, and often 

 seem to cover the whole surface: the other, more rapid and 

 energetic, when the animalcule darts forward in a particu- 

 lar direction, as if in pursuit of prey, and proceeds by sudden 

 and irregular starts, like a vivacious insect or fish. The vo- 

 luntary nature of their motions is evident from, the dexterity 

 they display in avoiding obstacles, while swimming together 

 in myriads in a single drop. 



The great agent in the movements of the animal frame 

 being the muscular fibre, it was natural to suppose that a 

 texture analogous to that of muscles might exist in these 

 latter genera of infusoria. It was not till very recently, 

 however, that the actual presence of contractile fibres could 

 be recognised. But this problem has at length been solved 

 by the discoveries of Ehrenberg, who, in his observations 

 of the larger and more highly organized species belonging 

 to the order of Rotifera, has, with a magnifying power of 

 380, distinctly seen muscular bands running in pairs between 

 the two layers of transparent membrane which envelope the 

 body. When the animalcule throws itself into its violent 

 lateral contortions, these fibrous bands are observed to be- 

 come broader and thicker, as well as shorter, on the side 

 towards w^hich the contractions take place. There can, 

 therefore, be no doubt that these are muscular organs, and 

 that they are the real agents by which the motions wit- 

 nessed are effected. 



These Rotifera, or wheel animalcules, ar-e so named from 

 {liu^j^ R their being provided with an apparatus 

 for creating a perpetual eddy, or circu- 

 lar current in the surrounding fluid. 

 The remarkable organs, by which this 

 effect is produced, are generally two in 

 number, (Fig. 80, r, r,) and are situ- 

 ated on the head, but do not surround 

 the opening of the mouth, as is the case 

 with the tentacula of polypes. They 

 consist of circular disks, the margins of 



