PROTOZOA. 



243 



Pio. IST.— A Cuinponnd Monad 

 (Uvella), X 1000. 



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 

 (as Yorticella), 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 

 among the distinctive features of these 

 Protozoans. Some have a pigment-speck 

 — the simplest sense organ — and in the 

 stem of Yorticella the first rudiments of 

 muscle may be found. Tiiey multiply so rapidly (cliiefiy 

 by self-division), that a Pararaecium, 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^viiith several hollow tentacles; and Ciliata, 

 which are furiiislied with numerous vibratile cilia. 



Fig. 1S8. — Infusorium 

 [Para'niRciUTtianrelia)^ 

 X 300: m, mouth; i), 

 contractile vesicles ; n, 

 nucleus. 



