164 NATURAL HlfeTORY 



A muscular system being the proper agent of volun- 

 tary contraction in the animal kingdom, its existence 

 might fairly be expected in the infusoria ; creatures so 

 remarkable for the rapidity and energy with which they 

 propel and translate themselves from one situation to 

 another. In respect to the former they can only be com- 

 pared with fishes ; in the latter, with insects. The mere 

 contractibility of tissue can never surely afford a sufficient 

 explanation of those active voluntary efforts by which they 

 avoid every obstacle, where myriads of creatures are 

 swimming in a single drop of water ; by which also they 

 convey nutriment towards their mouths, and perform the 

 acts of deglutition. 



From the extreme tenuity of the greater number of the 

 animalcules already described, and of all the Polygastrica 

 or the homogeneous Infusiora of Cuvier, no distinct mus- 

 cular fibres have been detected, although from their 

 vigorous contractions, as well as their presence in the 

 class Rotatoria, we may infer their existence. 



The superior size and diaphanous nature of this Vorti- 

 cella, enable us under the microscope to discern several 

 distinct bands of fibres of a greyish- white colour : that 

 these fibres perform the office of muscles is evident by 

 their contraction and dilatation : in contracting they be- 

 come shorter, broader, and more opaque on the contracted 

 side of the animalcule, and on the opposite side the 

 antagonist fibres elongate and almost vanish, in conse- 

 quence of their increased transparency. These facts, 

 and the position of these fibres, as shewn at m, w, o, p, of 



