358 bio way of the protozoa 



in which ex-cystment occurs and the freed organisms live and 

 multiply for a limited period. When their world dries up many of 

 the active organisms have encysted. Such cysts may be carried 

 with dust into food substances of man and other animals, and 

 through the agency of such contaminated food they are carried, 

 while remaining encysted, into the intestine where they do not 

 develop but which ultimately will provide a nutrient medium for 

 their development. In artificial cultures made up with feces of 

 different animals many such coprozoic Protozoa may be found, 

 and it is obvious that unwary observers may mistake them for 

 parasitic forms of the intestine. 



At the present time at least, it is hardly feasible to speak of a 

 definite coprozoic fauna since many of the cysts which pass through 

 an intestine may contain organisms capable of living in stagnant 

 waters, or as parasites in the intestines of different types of animals. 

 There are several forms of flagellates, however, which develop from 

 cysts in dung and which in the unencysted condition are not known 

 as parasites. Amongst such coprozoic flagellates perhaps the most 

 common type is Bodo caudatus, Duj., which, as would be expected, 

 is also common in sewage; Rhynchomonas nasuta, Stokes, is copro- 

 zoic in cockroaches (Parisi), but seems to be widely distributed in 

 fresh (Stokes, Belaf) and in salt water (Griessmann). Cercomonas 

 longicauda, Duj., from human feces also occurs in sewage. 



PARASITIC PROTOZOA. 



By virtue of protoplasmic irritability there is a constant reaction 

 of the organization to environmental stimuli (see Chapter V). 

 The reaction may be manifested by morphological or physiological 

 changes which we interpret as adaptations. If the stimuli are too 

 drastic the protoplasmic response is too vigorous and disintegration 

 results. A given stimulus or set of stimuli may result in controlled 

 reactions by one type of organization, while similar stimuli may be 

 fatal to other types. This principle is well illustrated by the proto- 

 zoan parasites where complete adaptation to the environmental 

 stimuli within a given host has resulted in organizations which dis- 

 integrate upon exposure to the different stimuli of a free-living 

 existence, and, vice versa, free-living forms are killed by the drastic 

 change to the conditions of an animal host. Great numbers of 

 species of Protozoa have become adapted to the specific environ- 

 ments of different animal hosts and no type of animal is free from 

 the possibility of protozoan infection. 



We can imagine a series of progressive adaptations whereby 

 free-living types may respond favorably to conditions of a partial 

 anaerobic medium (many such facultative aerobic forms are known). 

 Further adaptation to complete anaerobiosis is shown by the sapro- 



