44 



ANIMAL BIOLOGY 



ess is essentially destructive and the simplified products are rep- 

 resented in the relatively simple excretions of the Bacteria. 



But during bacterial ascendancy another factor has been grad- 

 ually intruding itself almost imperceptibly into the drama. This 

 is the microscopic animal life which has been multiplying with 

 increasing rapidity as conditions became more favorable, and 

 forthwith assumes the dominant life phase in the infusion. Among 

 the animal forms, the first to appear are exceedingly minute 

 flagellated Protozoa, known as Monads, many species of which 

 absorb products of organic disintegration brought about by the 

 Bacteria, while others exhibit holozoic nutrition, ingest solid 



Macronucleus- 



Myoneme fibres 



Myoneme of stalk 



Fig. 18. — Some types of Protozoa found in infusions. A, two species of 

 flagellated Monads (Mastigophora) ; B, Colpoda, a small Ciliate (Infusoria); 

 C, Vorticella, one of the most complex Ciliates. 



food — the Bacteria themselves. Then tiny ciliated Protozoa, 

 probably Colpidium and Colpoda, appear in untold numbers 

 and feed upon the Bacteria. The dominance of these smaller 

 Ciliates is brought to an end after a few days by the ascendency 

 of larger Ciliates which, though feeding to a certain extent upon 

 the already greatly depleted bacterial population, obtain most 

 of their food by eating the smaller Ciliates. And so the cycle of 

 life continues — saprophytic forms gradually being replaced in 

 dominance by herbivorous and these in turn by carnivorous or- 

 ganisms. In truth, nothing lives or dies to itself. (Figs. 18, 

 23, 26.) 



