INFUSORIA. 171 



smooth and equable gliding through the fluid, pro- 

 duced 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 particular direction, as if in pursuit of prey, 

 and proceeds by sudden and irregular starts, like 

 a vivacious insect or fish. The voluntary nature 

 of their motions is evident from the dexterity they 

 display in avoiding obstacles, while swimming toge- 

 ther 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 recognized. 

 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 Rotatoria, 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 become broader and thicker, as well as shorter, 

 on the side towards which the contractions take 

 place. There can, therefore, be no doubt that 



after being stretched to the utmost by the growth and multiphcation 

 of the monads within it, at length bursts and releases the included 

 smaller groupes of monads, which are found to be themselves 

 invested by their proper membrane, and which, in process of time, 

 undergo similar changes. 



