PROTOZOA. 



153 



really, understood. Many of them can endure drying, be blown 

 about in the spore stage, and then take up active life again 

 on the return of water, so that thereupon, in a few hours, In- 

 fusoria may literally swarm where none seemed to be. This 

 is responsible for the long life of the old belief that they arose 

 by " spontaneous generation," that is, without parents. It is 

 only in recent years that this belief has been finally disproved.. 

 It is known that they do not appear in water that has been 

 boiled and kept free from exposure to the air. 



FIG. 72. A compound Foraminiferan Nodosarla. a, aperture of shell; /, food 

 particles captured by the strands of protoplasm outside the shell; n, nucleus; sh, shell. 

 1-4, the successive chambers of the shell; i, being the oldest. 



Questions on the figure. Does this seem a colony or a single in- 

 dividual? Why? Why is digestion possible outside the capsule? Compare 

 this with figures of Protozoa in which there is no large aperture to the 

 shell. 



196. Classification of Protozoa. The following are the principal 

 classes of protozoa. 



Class I. Rhizopoda {root-footed'). Type: Amoeba. The Rhizopoda are 



amoeboid in form with pseudopodia, which may be either blunt (Fig. 



65) or slender (Fig. 72). The protoplasm may be naked (Amoeba) 



'or may secrete a shell either calcareous (Foraminifera) or siliceous 



(Radiolaria) . In the shelled forms the pseudopodia pass out through 



