PHYSIOLOGY OF THE MICROBES 101 



acid, then alkaline, just as in the stomach and small intestine 

 of mammals. 



The solid refuse of digestion is evacuated by an anus and the 

 liquid residue collects inside protoplasm in a little spherical 

 sac which from 

 time to time ex- 

 pels its contents 

 externally : this 

 latter is the con- 

 tractile vesicle 



which Ehrenberg 



, f . . FIG. 38. An amoeba expelling the residue of its 



took for the pul- food : various stages. (After Verwom.) 



sating heart. 



Oxygen is a primary necessity to protozoa as to bacteria : the 

 digestive vacuoles contain oxygen, the contractile vesicles 

 discharge carbonic acid, i.e., aerobic respiration. The protozoa 

 which live in surroundings deprived of free oxygen have, it is 

 certain, a method of respiration analogous to that of anaerobic 

 bacteria ; they draw their oxygen from reserve materials which 

 they have stored within themselves, e.g., glycogen. It is 

 believed, without being absolutely certain, that certain infusoria 

 can, like the intestinal worms (ascarides), break up glycogen 

 with the formation of valerianic and carbonic acids. Among 

 the products of excretion have been found uric acid and 

 phosphate of lime (Schewiakoff). Excretion is a fairly active 

 process since the vacuole contracts often (every four to eighteen 

 seconds according to temperature in Stylonychia pustulata). 

 According to Maupas the infusoria discharge during a space 

 of time which varies from two to forty-six minutes a volume of 

 liquid equivalent to the whole volume of the animal. 

 Stimulation from the exterior is always accompanied in the 

 protozoa by a manifestation of energy : they possess irritability. 

 The excitant may be a touch, a ray of light, heat, electricity or 

 a chemical substance, and the protozoon in its reactions to the 

 stimulus acquires habits such that its physiology is full of as 

 many problems as that of the higher animals, not excepting 

 problems in psychology. Although possessing neither nervous 



