196 ANIMALS 



two stages there is a stage when this substance is invisible in 

 solution, and it is during this stage that it passes from the 

 vacuole into the protoplasm. 



453. Let us recall the phenomena of fermentation as exhib- 

 ited by the yeast plant or bacteria. Here we have living cells en- 

 closed in a membrane, through which no visible particle is known 

 to pass. The bacteria live in a watery medium surrounded 

 by solid substances, upon which they are nourished. In the 

 medium there appear substances which are secreted by the liv- 

 ing cells and which act upon the food substances in such a way 

 as to cause them to go into solution. This process may come 

 about in many different ways, but the result is always a solution 

 which may be absorbed by the cell through the membrane. 

 The chief difference between the bacteria and the amoeba, with 

 respect to the way in which the food is prepared, so that it 

 may be absorbed, is this: The bacteria fill the surrounding 

 medium with an enzyme which dissolves the food substances 

 there. The amoeba takes into its body a droplet of the medium 

 containing a particle of food, and into this droplet of the medium 

 it secretes a digestive fluid. We may transpose terms and say 

 that the bacteria digest the food before it is taken into the 

 body and the amoeba carries on a process of fermentation 

 within its food-vacuoles. That is to say, digestion is a matter 

 of fermentation. 



454. The term gastro-vascular cavity applied to the central 

 cavity of hydra indicates that it is analogous to the stomach, 

 and hence, concerned in digestion. The food unquestionably 

 passes into this cavity, but to what extent it is there digested is 

 uncertain. Small particles are known to be captured by the 

 flagellate cells of the entoderm and engulfed by the protoplasm. 

 So that in this case as well as in Sponges and some Flat-worms 

 the digestion resembles that of amoeba. In this case the func- 

 tion of the gastro-vascular cavity would be to serve as a sort of 

 trap for the food particles. But frequently objects are captured 



