ECOLOGY, COMMENSALISM AND PARASITISM 385 



but it is also possible that they may find under such conditions a 

 more suitable environment for growth and reproduction. Species 

 of Bodo which are amongst the commonest coprozoic flagellates 

 have been observed in the urine (Powell and Kohigar, 1920). The 

 intestinal flagellates, particularly Embadomonas intestinalis, Chilo- 

 mastix mesnili, Tricereomonas intestinalis, Trichomonas hominis and 

 Giardia intestinalis are usually present in large numbers in diarrheic 

 stools while only cysts, as a rule, are found in normal stools. This 

 certainly suggests an etiological connection, particularly with 

 Giardia infections in which periodic attacks of diarrhea occur with 

 passing of quantities of clear mucus in which the flagellates are 

 abundant. 



Parasitic Rhizopods.- While Sarcodina are perhaps less striking in 

 their adaptations than are other groups of Protozoa, they are, 

 nevertheless, more or less specialized in conformity with their 

 habitats and modes of life. The fundamental type is spherical and 

 characteristic of suspended or floating forms (Heliozoa and Radio- 

 laria), but adaptations serving a hydrostatic purpose are numerous, 

 particularly in the great group of Radiolaria. Creeping forms are 

 found in superficial slime of ponds and sea or on stalks and leaves 

 of water. plants and are more or less segregated in localities where 

 appropriate food is abundant. Thus Amoeba vespertilio may be 

 found in fresh water where diatoms and algae are abundant: 

 A. proteus in waters with decomposing organic matter rich in bac- 

 teria, or Pelomyxa palustris in still fouler waters. Amoeba terricola, 

 many testate rhizopods and related forms are more terrestrial, living 

 in moss or damp earth and sand ; here also may be found the major- 

 ity of Mycetozoa, especially on damp and decaying wood. In 

 short, there are few damp places that are devoid of ameboid types. 



The Sarcodina are never as spectacular as the Mastigophora or 

 Sporozoa in their adaptations for parasitism, but many types have 

 become adapted to the semifluid habitats of plant and animal hosts 

 or to the more fluid environments of animal digestive tracts. Copro- 

 zoic forms are not uncommon, many types, like coprozoic flagel- 

 lates, passing through the digestive tract while encysted to develop 

 later in the dejecta (e. g., Dimastigamrba, Sappinia species). Con- 

 versely the true parasites are active only in the lumina of the 

 alimentary tract and are able to withstand the rigors of an external 

 life only when protected by cysts. Such cysts, through contamina- 

 tive infection, germinate in the digestive tract where some types 

 of Endameba cause acute or chronic intestinal diseases. 



Many amebae are ectoparasitic. One, Amoeba hydroxena (Entz, 

 1912), occurs on hydra (H. oligactis); another, A. pacdophora, Caul- 

 lery (1906), on the eggs of a crustacean Peltogaster curvatus; .1. 

 m/udcola, Chatton (1909), occurs on the gills of marine fish. Protista 

 are not exempt— species of Svhaerita parasitize Euglenoids, I olvox, 

 25 



