86 AN INTRODUCTION TO THE STUDY OF FOSSILS 



of soluble peptones similar to the products of proteid digestion 

 in the higher animals. This is followed by a digestive ferment 

 like trypsin, acting in an alkaline medium. 



The manufacture of the acid secretion is intimately con- 

 nected with the chromatin material of the nucleus, for digestion 

 does not take place when the nucleus is absent. '' Protozoa 

 evidently have the power of secreting different kinds of ferments 

 in response to the stimulus of different kinds of living food 

 particles." Some types of Protozoa can create starch-dissolv- 

 ing ferments or fat-emulsifying ferments, though other species 

 throw the starch grains out of the body with other indigestible 

 parts of the food (13). 



Assimilation. — '^ The granules that are formed by the break- 

 ing down of the food particles through the digestive process are 

 ultimately distributed by means of the protoplasm which 

 streams to all parts of the protozoon " through the animal's 

 rolling method of progression and by osmosis. " Some are 

 probably converted directly into protoplasm by an assimilative 

 process that is as little understood in these forms as in the 

 Metazoa " (13). 



Excretion. — The undigested food particles pass outwards 

 into the surrounding water, and are left behind as the amoeba 

 rolls forward. Thus, whatever part of the amoeba's body 

 touches food becomes mouth, whatever portion the food particle 

 enters becomes digestive cavity, and the undigested material 

 may leave from any part of the surface. 



It is probable that the contractile vacuole is the chief organ 

 of excretion, that by its means the animal rids itself of the 

 waste products of assimilation and metabolism, such as urea 

 and carbon dioxid. 



Respiration, or the exchange of gaseous waste products for 

 oxygen, takes place through the entire surface of the body, 

 most of the oxygen being obtained in this way, though probably 

 most of the carbon dioxid is thrown off by the contractile 

 vacuole. 



