ECOLOGY, COMMENSALISM AND PARASITISM 361 



even if not acceptable for trypanosomes, is certainly given by the 

 life histories of several diverse types of the protozoan parasites. 

 Amongst flagellates, for example, the genus Trichomonas is one of 

 of the most widely distributed types in man. While there is some 

 question of the identity of species, representatives of the genus have 

 been recorded from the human mouth (many observers), from the 

 toe-nails (Wenyon), from sputum (many observers), from the 

 pleural cavity, from the vagina (many observers), in urine (many 

 observers), etc. Wenyon (1920) demonstrated the passage of forms 

 from the intestine into the surrounding tissue, and Pentimalli (1923) 

 found them in the blood. Similarly the widely distributed entozoic 

 genus Giardia and other flagellates — Eutrichomastix, Octomitus, 

 etc., are frequently present in great numbers in the blood (Reich- 

 enow). Even more striking instances of adaptation from entozoic 

 to hematozoic mode of life are shown by coccidimorpha amongst 

 the Sporozoa. Here in Shellackia, Lank ester ella, Hepatozoon, etc., 

 infection is contaminative and blood parasitism is developed in 

 varying degrees. In all of these, infection is by the contaminative 

 method, the sporozoites of Shellackia and Hepatozoon develop and 

 reproduce like typical coccidia in epithelial cells of the gut. In 

 Hepatozoon (Miller) the gametocytes enter the blood where they 

 are engulfed by phagocytes. These are eaten by the mite Lelaps 

 echidninus, fertilization takes place in the gut and sporozoites are 

 formed in the body tissues of the mite— the latter when eaten by 

 a rat enter epithelial cells and repeat the cycle. In Shellackia 

 there is a similar history, but macrogametes penetrate the gut wall 

 of the host — a lizard— and are fertilized in the deeper tissues where 

 sporozoites are formed. These make their way into the blood where 

 they enter red blood cells or leukocytes. Here they remain dor- 

 mant until eaten by a mite and the mite eaten by a lizard. In the 

 lizard the cycle is repeated. Lankesterella, a blood parasite of the 

 frog, is a typical hematozoon. Here the initial sporozoite stage is 

 a gut parasite of the frog, eaten with infected leeches (Hemiclepsis 

 marginata). Unlike the other forms mentioned, no development 

 occurs in the frog's gut but the sporozoites penetrate the gut wall 

 and enter the blood where, as intracorpuscular parasites, they grow 

 and reproduce as hematozoa. 



While the above cases illustrate the change from an entozoic to 

 a hematozoic mode of life in the same individual they do not 

 cover the whole story of the blood parasites. It is perfectly possible 

 for a gut parasite of one animal to become a blood parasite in an 

 entirely different type of animal. This indeed was regarded by 

 Leger (1904) as the mode of origin of mammalian trypanosomes. 

 Developing as entozoic parasites of insects they were inoculated 

 when the insect began to feed on mammalian blood and, finding a 

 suitable medium for growth and reproduction, they multiplied until 



