HOST-PARASITE RELATIONS! INTESTINAL PROTOZOA 



Amoeba proteus feeds on minute aquatic plants, other 

 protozoa, bacteria and other animal and vegetable matter 

 that it encounters in fresh water, whereas Endamoeha 

 coli feeds on bacteria and animal and vegetable matter 

 that occur in the contents of the intestine. Food vacuoles 

 are formed in both species into which digestive juices are 

 secreted from the surrounding cytoplasm and in which 

 digestion takes place, the digested material being assimi- 

 lated and the undigested material extruded through the 

 surface of the body. Respiration takes place through the 

 general body surface. The waste products of metabolism 

 are excreted through the ectoplasm. The only striking 

 difference between the two species morphologically and 

 physiologically is the presence of a contractile vacuole 

 in Amcsha proteus and its absence in Endamoeha coli. 

 The functions of the contractile vacuole are supposed to 

 be principally respiratory and excretory, — functions that 

 in parasitic species are satisfactorily performed through 

 the surface of the body. 



The habitats in which Amoeba proteus and Endamoeba 

 coli live differ in many respects. The factors of the en- 

 vironment of the former are well known to every student 

 of biology but not so those of Endamoeba coli. This 

 parasitic species has for its habitat the lumen of the large 

 intestine. Here it lives in total darkness in the liquid con- 

 tents, which consist of digested food substances, bacteria 

 of various sorts, the products of bacterial decomposition 

 and more or less changed secretions from the digestive 

 glands. This medium is more viscid than water and 

 chemically much more complex. The temperature is rela- 



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