360 BIOLOGY OF THE PROTOZOA 



reactions of the host, as well as toward the perpetuation of the 

 species. Many of them are obligatory parasites of specific animal 

 types and must find their appropriate environment to live. The 

 first question that arises is: How do they get into the body? As 

 a matter of fact the host, for example, the human body, is fairly 

 well protected and gateways to the insides are limited practically 

 to the mouth, nasal passages and the skin. The most obvious of 

 these is the mouth leading into the digestive tract, and infection may 

 follow the intake of contaminated food and drink. By far the 

 greatest number of protozoan parasites are introduced by this con- 

 taminative method. Minute germs and cysts may be taken in 

 with air currents through the nose and throat, but this method is 

 mainly limited to bacterial infections and if we exclude the ques- 

 tionable Chlamydozoa, protozoan infection by this method is prac- 

 tically unknown. 



While probably the majority of endoparasitic Protozoa are 

 harmless, others are pathogenic and in each group with the probable 

 exception of the ciliates we find gradations between the two, while 

 with the Hvpermastigida and termites we find a perfect symbiosis 

 (p. 199). 



The skin is a most effective barrier against infection and so long 

 as it is kept in good condition infection by this means is reduced 

 to a minimum. Abrasions, hang-nails, casual cuts, etc., however, 

 are portals of entry and bacteria, spirochetes or small flagellates 

 may gain access to the blood through such injuries. Or the skin 

 may be punctured by biting bugs, arachnids, flies, mosquitoes, 

 leeches and the like, and disease germs may be transmitted in this 

 way. Scratching the skin at points of irritation, thereby providing 

 entrance for possible parasites deposited with feces by ticks, mites 

 or other ectoparasites, is another means of inoculative infection. 

 Only rarely do Protozoa have invasive power of sufficient strength 

 to penetrate the unbroken skin and then only in the more delicate 

 coverings; such a disease is the so-called horse syphilis caused by 

 Trypanosoma equ i perdu m. 



Obviously the most important of these modes of infection is that 

 by contaminated food and water taken into the digestive tract 

 through the mouth. Once adapted to the conditions of the gut, 

 intestinal parasites are prepared for further explorations and adap- 

 tations which may lead to parasitism in various organs of the host. 

 According to their seat of parasitism, internal parasites may be 

 grouped as entozoic (gut-dwelling), celozoic (lumen-dwelling), 

 hematozoic (blood-dwelling), cytozoic (intracellular), histozoic 

 (tissue-dwelling), karyozoic (intranuclear), etc. 



In connection with the life history of trypanosomes Minchin 

 (1908) developed the thesis that hematozoic forms originate from 

 entozoic parasites in the same host. Support for this point of view, 



