THE INFUSOKIA. 



107 



substances, occupy the highest rank in the Protozoic world. 

 They are all exceedingly minute, but of various dimensions ; 

 the greater number being individually in visible to the naked eye, 

 while some, like the Stentor, attain the comparattvely large 

 size of l-30th of an inch. This beautiful creature resembles a 

 gelatinous trumpet, and is flexible and contractile in all direc- 

 tions, either while swimming about freely in the water, or while 

 attached, as it frequently is, to some foreign body by means of 

 a little sucking disk which terminates the pointed extremity of 

 the tail. 



The various species of Infusoria exhibit a great diversity 

 of form globular, oval, cylindrical, thread-like. Most of them 

 are free in their movements, some permanently attached to 

 stalks; by far the greater number are colourless and trans- 

 parent, while some have a yellowish, greenish, or reddish tinge. 

 Compared with the Pro- 

 tozoic families already 

 described, their higher 

 organic development is 

 chiefly exhibited by their 

 possession of a mouth and 

 rudimentary digestive or- 

 gan, while well-marked 

 cilia or hair-like append- 

 ages,disposed either along 

 the entire margin of the 

 body, as well as around the 

 oval aperture, or limited 

 to the immediate vicinity 

 of the mouth, serve them as instruments of prey and loco- 

 motion. During the life of the animal these cilia are in 

 almost constant action, their motion consisting of bends in 

 rapid succession from base to point, and of an immediate 

 return to the original position, not unlike the undulating 

 motion of a cornfield under the influence of the wind. 



Thus currents or vortices are produced which enable their 

 tiny possessors to ingulf the still more tiny prey that comes with- 

 in reach of their irresistible whirlpools. The exceeding minute- 

 ness, as well as the rapid movements of the cilia, often make it 

 difficult to observe them, though, when invisible, their existence 



Various Forms of Infusoria. 

 . Coleps hirtus. 3, 4. Trachelms anas. 5. Tra- 



chelms ovum. 

 o, mouth ; a, outlet of alimentary canal. 



