INFUSORIA. 



183 



as usual, excited by the vibrations of the cilia ; 

 which in the simpler species, such as the 

 17 Vorticella ci/athina, here deli- 



neated, are the efficient in- 

 struments of progressive mo- 

 tion. When attached by its 

 foot-stalk, the vorticella ad- 

 vances in search of food, by 

 the extension of the foot-stalk 

 into a straight line ; but quickly 

 retreats from danger, by suddenly throwing 

 it into spiral folds. Many of the species of 

 vorticellae are so exceedingly diminutive as to 

 be imperceptible without the aid of the micro- 

 scope. They conduct us, therefore, by a natural 

 gradation, to the next order we have to notice, 

 and which is composed wholly of microscopic 

 animals. 



§ 4. Ijifusoria. 



The Infusory animalcules, or Infusoria, were so 

 named by Muller, a Danish naturalist, from the 

 circumstance of their swarming in all infusions 

 of vegetable or animal substances that have 

 been kept for a sufficient time. They are, in 

 general, far too minute to be perceptible to the 

 naked eye : it is to the microscope alone, 

 therefore, that we owe our knowledge of their 



