TENANTS OF A DROP OF WATER. 123 



leaving it for a few days exposed to the influence of 

 the sun and air, when the infusion will be found 

 swarming with animalcules. A drop of this infu- 

 sion, or of water from any pond or ditch, if put 

 upon a thin slip of glass, made for the purpose, and 

 viewed through a good microscope, will then present 

 an astonishing spectacle. A little practice in the 

 management of the instrument, and also of the eye, 

 is necessary ; for experience greatly improves the 

 eye in the use of the microscope. It is important 

 to observe, that as these creatures were first dis- 

 covered in infusions, and were supposed to be re- 

 stricted to them, they were primarily denominated 

 Infusoria. 



Some microscopic animalcules are termed Poly- 

 gastrica, from their being supposed to possess a 

 number of internal sacs, generally regarded as 

 stomachs, and which are easily seen through the 

 transparent covering of the animals, when viewed 

 under a powerful microscope. No nervous filaments 

 are discernible in their composition ; nor any brain 

 or central mass to which the impressions received 

 from external agents can be transmitted. It is 

 supposed that the nervous matter, instead of rami- 

 fying like threads through the system, is diffused 

 universally throughout their composition, and is 

 blended with it ; but this theory is not positively 

 proved, although probability favours it. Still, each 

 one is fully prepared to enjoy existence. So far 

 from forming a solitary class, twenty-six species of 



