160 



THE ANIMAL KINGDOM 



Figure 8.11. The internal, siliceous skeleton of a radiolarian. (E. Giltsch, Jena.) 



in rapid succession, these species of nummulites. From these the driller 

 can estimate just how far into the paleozoic deposit he has drilled. This 

 is one of the few instances where an industry uses the services of a 

 taxonomist— in this case a specialist on the classification of one family 

 of extinct protozoa. 



50. Class Ciliata 



Ciliates can be distinguished from the flagellates and rhizopods not 

 only by their cilia, but also by their nuclei. Each ciliate has two nuclei, 

 a large macronucleus which governs the ordinary activities of the cell, 

 and a small micronucleus which functions during sexual reproduction. 

 Both nuclei divide at each mitosis, but at sexual reproduction the 

 macronucleus disintegrates, and the micronucleus gives rise to both 



