IX. 



THE BELL- ANIMALCULE (Vorticdla). 



THE great majority of those animal organisms which are 

 more complex than Amoeba, begin their existence as simple 

 nucleated cells, having a general similarity to Amoeba; and 

 the single nucleated cell which constitutes the whole animal 

 in its primitive condition divides and subdivides until an 

 aggregation of similar cells is formed. And it is by the 

 differentiation and metamorphosis of these primitively simi- 

 lar histological elements that the organs and tissues of the 

 body are built up. But in one group, the Infusoria, the 

 protoplasmic mass which constitutes the germ does not 

 undergo this process of preliminary subdivision, but such 

 structure as the adult animal possesses is the result of the 

 direct metamorphosis of parts of its protoplasmic substance. 

 Hence, morphologically, the bodies of these animals are the 

 equivalents of a single cell; while, physiologically, they may 

 attain a considerable amount of complexity. 



The Infusoria abound in fresh and salt waters, and make 

 their appearance in infusions of many animal and vegetable 

 substances, their germs either being contained in the sub- 

 stances infused, or being wafted through the air. Their 

 diffusion is greatly facilitated by the property which many 

 of them possess of being dried, and thus reduced to the 

 condition of an excessively light dust, without the destruc- 



