PROTOZOA. 



Pis. 187. A Coiuponnd Monad 



CLASS IV. Infusoria. 



This unassorted group of living organisms 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 (uua), x 1000. 



(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 no cellular 

 tissues, but the whole body is a sin- 

 gle cell), covered by a membrane, or 

 skin, having one or two contractile cavi- 

 ties, and a nucleus. Food-granules can 

 often be seen. On one side is a slight 

 depression, or " mouth," leading to a 

 short, funnel-shaped throat. A mouth 

 and a rudimentary digestive cavity are 

 amon> the distinctive features of these 



FIG. 1S8. Infusorium 



Protozoans. Some have a pigment-speck 



X 300: m, mouth; v, . . , , . - 



contractile vesicles ; n, the simplest sense organ and in the 



DUCleas> stem of Yorticella the first rudiments of 



muscle may be found. They multiply so rapidly (chiefly 



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



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



There are three main groups: Flagellata, or Monads, 



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



Tentaculifera, with several hollow tentacles; and Ciliata, 



which are furnished with numerous vibratile cilia. 



