CHAPTEE X. 



THE PATHOGENIC RHIZOPODA. 



Thk biological conditions which underlie parasitism are but little 

 known, but, as with free-living protozoa, the dominant factor is the 

 problem of food-getting. The causes which lead an organism to 

 invade a specific organ or tissue must, in the final analysis, be traced 

 to this function, and reproduction leading to complete annihilation 

 a cell or group of cells follows a parasite's life in a suitable food 

 medium. There is a limit also to the kinds of parasites that can become 

 cell-infesting forms, for the organism must have either the mechanical 

 or cytolytical power of breaking down the barriers of a cell, and 

 physical force enough and of a certain kind, to enable it to penetrate 

 the cell membranes and cytoplasm. For such a function cilia are not 

 useful, nor flagella, and we find that ciliates and ordinary flagellates 

 rarely become intracellular parasites, and then only after losing their 

 motile organs; unless, as in trichonympha, pyrsonympha, etc., they 

 are provided with special anterior boring organs, by which they pene- 

 trate the cell membranes, or unless, as in spirocheta, they possess the 

 power of undulatory motion independent of flagella action (Fig. 114). 

 Spirochetes may thus become cell-dwelling as well as fluid-dwelling 

 forms, and some, like Sp. microgyrata or Treponema pallidum, work 

 their way through the tissues of an infected host and not infrequently 

 bore into the cells themselves. The ciliated and flagellated protozoa, 

 however, are typically fluid-dwelling forms, and when they attack the 

 epithelial cells of an organ it is usually only for purposes of attach- 

 ment, as in trichonympha and pyrsonympha. There is considerable 

 evidence, however, to indicate that one of the ciliates, balantidium, is 

 occasionally found inside the mucosa of the intestine, and even within 

 the muscular coating of the colon, while collections often appear in the 

 epithelial cells and, apparently, cause the ulcers that are found there. 

 Two kinds of these ciliated parasites are common in man, Balantiditim 

 coli, frequent in the rectum, and Bal. minutum, and, according to Strong, 

 Brooks, and Stengel, with others, the parasite becomes an important 

 etiological factor in catarrhal inflammation of the intestine (Fig. 115). 

 Other observers, including Malmsten, Opie, Doflein, and others, hold 

 that these forms are quite harmless, increasing in number with dis- 

 orders of the digestive tract, and for this reason are not uncommon in 

 the intestinal tract of victims of cholera, typhoid, dysentery, or diar- 



