1 82 Eletnentary Zoology. 



absolute criterion for determining whether a given unicellular 

 or few-celled organism is a plant or an animal. Hence it 

 follows that there are organisms which have been tossed about 

 from the vegetable to the animal sub-kingdom, and back again. 

 To these doubtful organisms the term " Protista " was applied 

 by Hseckel. An organism known as Englena will serve as an 

 instance. It moves about freely by means of a flagellum, and 

 has an cesophageal tube like a Vorticella, and therefore feeds 

 like an animal; but it has also chlorophyll, and at times 

 becomes encysted, the cell-wall being composed of cellulose. 

 There is not, however, much classificatory advantage to be got 

 by using the term " Protista ; '' for it only doubles the difficulty. 

 We have, without this division, only to distinguish between 

 animals and plants ; with it, between animals and Protista, and 

 between plants and Protista. The facts simply emphasize the 

 common descent of all living beings from a common stock. 



