PROTOZOA. 



243 



FIG. 1ST. A Compound Monad 

 (Uvella), X 1000. 



CLASS IV. Infusoria. 



This unassorted group of living particles derived its 

 name from the fact that they were first discovered in veg- 

 etable infusions. Every drop of 

 a, stagnant pool is crowded with 

 them. They are all single and 

 microscopic, yet of various sizes, 

 the difference between the small- 

 est and largest being greater than 

 the difference between a Mouse 

 and an Elephant. Some are fixed 

 (as Vorticdla), but the majority are free, and constantly 

 in motion, propelled by countless cilia, as a galley by its 

 oars. The delicate body consists of two 

 layers of sarcode (there are na cellular 

 tissues, but the whole body represents a 

 single cell), covered by a membrane, or 

 skit*, having one or two contractile cavi- 

 ties> and a nucleus. Food -granules can 

 often be seen-. On one side i& a slight 

 depression, or " mouth," leading to a 

 short, funnel-shaped throat. A mouth 

 and a rudimentary digestive cavity are 

 among the distinctive features of these 



1SS. Infusorium 



(Parameciumauniia), Protozoans. Sotne have a pigment-speck 



X 300: m, mouth; t>, ^ . , , , . , 



con tractile vesicles ; n, the simplest sense organ and m the 



uucleus - stem of Vorticella the first rudiments of 



muscle may be found. They multiply so rapidly (chiefly 



by self-division), that a Pararnecium, the most common 



form, may become the parent of 1,364,000 in forty-two days. 



There are two main groups: Flagellata, or Monads, 



provided with one or two flagella, or long, bristle -like 



cilia; and Ciliata, which are famished with numerous 



vibratile cilia. 



