14 THE SCIENCE OF LIFE. 



M Ciller, the father of modern physiology, substituted 

 the term " organic force" for that of "animating princi- 

 ple," and Dr. Prout used the term " organic agent." 

 The precise term employed is of but secondary import- 

 ance compared with the dualistic conception, which is 

 quite satisfactory to the large majority of thinkers. 



6. We shall be able to appreciate this subject better 

 if we consider the life-history of some simple animal. 



It is well known that infusions of vegetable or animal 

 substances contain many living forms of extreme sim- 

 plicity of structure, called Infusoria. Many such are 

 found in ponds, or running water, or in the sea. A very 

 beautiful kind of Infusoria, common among half-decayed 

 leaves, has received the name of Vorticella, or bell- 

 shaped animalcule. There are several species, the most 

 common being known as Vorticella nebulifera. Take up 

 from a pond a little twig, covered with mold or mucus- 

 like substance, and place it under the microscope. In 

 all probability you will see a colony of Vorticellae, 

 (Fig. i.) 



Each animalcule has a glassy, transparent bell, with a 

 thick lip or rim, fringed with cilia or hair-like projec- 

 tions. These cilia are sometimes withdrawn, but when 

 active vibrate rapidly, so as to make a sort of whirlpool 

 in the water, in the vortex of which smaller animals or 

 vegetables may be conveyed as food to the interior of 

 the Vorticella. A number of pellucid spots may be 

 seen in the body of each animalcule, which were for- 

 merly regarded as stomachs. Professor Ehrenberg, who 

 elaborately investigated this class of animal life, gave 

 the name Polygastrica (many stomachs) to those animal- 



