PROTOZOAN FAUNA OF HAY INFUSIONS 223 
When the infusions are first made up, the liquid, though col- 
ored, is transparent, but within forty-eight hours it becomes 
markedly turbid due to the development of countless bacteria. 
The bacteria at this time are equally distributed throughout the 
medium but on the third day a ‘zoogloea’ begins to be established 
and gradually increases in amount until it finally falls to the bot- 
tom and another is formed. In some cases, however, the ‘zoo- 
gloea,’ after reaching its maximum thickness, at approximately 
the end of thirty days, gradually thins out and practically dis- 
appears in situ. These variations in the transformation of the 
‘zoogloea’ introduce a complicating factor in the study of the 
protozoan life of infusions, because in the cases in which it falls 
to the bottom, it changes the center of population of certain 
types quite suddenly, and thus causes a redistribution of some 
forms. Thebacteria, then, at first are equally distributed through- 
out the fluid, then the largest number is at the bottom and top, 
while in the center of the volume of liquid there are comparatively 
few. The hay and smaller amount of oxygen at the bottom, and 
the more abundant supply of oxygen at the top, offer attractions 
for different forms with the result that apparently approximately 
the same number are to be found in each region. After the 
‘zoogloea’ has fallen or disappeared the center of bacterial life 
is again at the bottom amongst the remnants of the disintegrating _ 
hay. 
As soon as the bacteria have become numerous, and _ their 
action on the hay has put a certain amount of it in a form avail- 
able for animal life, then occurs the great growth of Protozoa, 
comprising saprophytic, herbivorous, carnivorous and omnivorous 
forms, and this phase of the life of the infusions we shall consider 
in detail. 
After the period of greatest protozoan fauna has passed, roti- 
fers become numerous, and as the diatoms, desmids, and filamen- 
tous cyanophyceae and chlorophyceae flourish, under proper 
conditions of illumination, several species of Anguillula, copepods, 
etc., are more or less abundant. This condition of the fauna and 
flora merges imperceptibly into what may be called a condition of 
nearly stable equilibrium, in which green plants and animals, under 
