HABITAT RESTRICTIONS 



Host-parasite specificity. This phenomenon, which is 

 one of the most interesting characteristics of animal 

 parasites, is fully discussed later (see page 45). 



Habitat restrictions. The situation as regards free-liv- 

 ing protozoa is quite similar. A particular species has its 

 optimum habitat ; it may even exist although the environ- 

 mental factors depart considerably from the optimum. 

 There is, however, a point beyond which existence is 

 impossible; life in a habitat where this point has been 

 reached for one or more factors therefore ceases and the 

 species concerned cannot grow and multiply in such a 

 habitat, no matter how frequently and abundantly speci- 

 mens may be introduced into it. Many of the terms 

 familiar to parasitologists might with equal force be 

 applied to free-living protozoa; for example, a tolerant 

 host or habitat is one in which a protozoon can live and 

 multiply successfully, whereas in a refractory host or 

 habitat life and multiplication are difficult; a "natural" 

 host or habitat is one in which a certain protozoon gen- 

 erally is to be found in nature, whereas a host or habitat 

 in which a species usually does not live is considered 

 "foreign." Conditions are similar as regards accidental 



Plate i 



Amceb^ Living in Man 



(All figures magnified about 2000 diameters) 



la and ib. Endanusba histolytica. la, trophozoite. ib, cyst. (After 

 Dobell.) 



2a and 2b. Endanweba coli. 2a, trophozoite. 2b, cyst. (After Dobell.) 



3a and 3b. Etxdolimax nana. 3a, trophozoite. 3b, cyst. (After Taliaferro 

 and Becker.) 



4a and 4b. lodamccba williamsi. 4a, trophozoite. 4b, cyst. (After Talia- 

 ferro and Becker.) 



5. Endamoeba gingivalis. (After Dobell.) 



6. Dientatnoeba fragilis. (After Taliaferro and Becker.) 



15 



