PHYLUM PROTOZOA. ONE-CELLED ANIMALS 



35 



are attached to some solid object. At certain 

 times any animal or plant that is not too 

 large may be ingested, but when several 

 species are present, selection is evident, since 

 the small flagellate Chilomonas (Fig. 21) is 

 engulfed more readily than the larger ciliate 

 Colpidium; and the flagellate Monas is rarely 

 taken if Chilomonas or Colpidium is avail- 

 able. As many as 50 to 100 chilomonads may 

 be ingested in a single day. When amoebas 

 are fed exclusively on chilomonads, they 

 grov/ and multiply for a few days but soon 

 die; whereas when fed exclusively on Colpi- 

 dium they grow large, become sluggish, and 

 multiply slowly, but do not die. The amoeba 

 may live for 20 days or more without food, 

 but it decreases in volume until it is only 

 about 5 per cent of its original size. 



Digestion 



The food vacuole (food chamber) (Fig. 

 11) serves as a sort of temporary stomach. 

 Digestive fluids (enzymes) are secreted into 

 it by the surrounding cytoplasm. The con- 

 tents are at first acid and then become 



alkaline. In man, as we shall see later, food 

 materials encounter an acid medium in the 

 stomach and an alkaline medium in the in- 

 testine. Chilomonads remain alive in the 

 food vacuoles from 3 to 18 minutes and are 

 digested in from 12 to 24 hours. Proteins, 

 fats, sugars, and starches are broken down. 

 The digested material diffuses out of the 

 vacuoles into the cytoplasm, with the vac- 

 uole decreasing in size until only indigesti- 

 ble matter remains. This is eventually 

 eliminated. 



Egestion 



Indigestible and sometimes partially di- 

 gested particles are egested at any point on 

 the surface of the amoeba, there being no 

 special opening to the exterior for this waste 

 matter. Usually such particles are heavier 

 than the cytoplasm of the amoeba; and as 

 the animal moves forward, they lag behind, 

 finally passing out at the end away from the 

 direction of movement; that is, the amoeba 

 flows away, leaving the indigestible solids 

 behind (Fig. 14). 



New cell membrane 



Figure 14. Amoeba verrucosa. Part of a specimen showing three stages in the egestion of an 

 indigestible particle; development of a new cell membrane prevents loss of endoplasm. (After 

 Howland.) 



Assimilation 



The digested material absorbed into the 

 cytoplasm is built up into protoplasm, that 

 is, it is assimilated, and growth results. 



Dissimilation ( catabolism ) 



The energy for the work done by the 

 amoeba comes from the breaking down of 

 complex molecules of protoplasm by oxida- 



tion or physiologic burning. The products 

 of this slow combustion are the energy of 

 movement, heat, and residual matter. Ordi- 

 narily the residual matter consists of solids, 

 fluids consisting mainly of water, some min- 

 eral substances, urea, and carbon dioxide. 

 Thus it will be seen that the products of 

 respiration are included in this residual 

 matter. 



