Protozoans 



159 



Protozoans — One of the best known animals in the 

 world, one that is pedagogically exploited in every 

 biological laboratory, is the Amosba (fig. 69a). Plastic, 

 ever changing in form and undifferentiated in parts, 

 this is the animal that is the standard of comparison 

 among things primitive. Its name 

 has become a household word, and an 

 every-day figure of speech. A little 

 living one-celled mass of naked pro- 

 toplasm, that creeps freely about 

 amid the ooze of the pond bottom, 

 and feeds on organic foods. It 

 grows just large enough to be recog- 

 nized by the naked eye when in most 

 favorable light, as when creeping up 

 the side of a culture jar: on the 

 pond bottom it is undiscoverable 

 and a microscope is essential to 

 study it. 



Related to Amoeba are several 

 common shell-bearing forms of the 

 group of Sarcodina (Rhizopoda) 

 that often become locally abun- 

 dant. Difflugia (fig. 69c) forms a 

 flask-shaped shell composed of mi- 

 nute granules, that, magnified, look 

 like grains of sand stuck together 

 over the outside. The soft amoeba-like body protrudes 

 in pseudopodia from the mouth of the flask, when travel- 

 ing or foraging, or withdraws inside when disturbed. 

 Arcella (fig. 69^) secretes a broadly domeshaped shell, 

 having a concave bottom, in the center of which is the 

 hole whence dangle the clumsy pseudopodia. One 

 species of Arcella, shown in the following figure, has 

 the margin of the shell strongly toothed. Both of 

 these genera, and other shell-bearing forms, secrete 



