1922] on Microscopic Parasites and their Carriers 507 



strains of parasites, subjects of great interest into which there is no 

 time to enter now. Though the majority of natural hosts may 

 suffer little, owing to the delicate mechanisms which render such a 

 balance possible, occasionally a natural host is defective or the strain 

 of parasite introduced is more virulent, so that death results rapidly 

 and host and parasite are destroyed. Such exceptions are disadvan- 

 tageous to host and parasite alike, but do uot invalidate the general 

 principles underlying the relationship of host to parasite. 



The microscopic animal parasites, or protozoa, live and multiply 

 in the body of their hosts ; it may be in the lumen of the intestine, 

 in the blood stream, or actually in the tissues. It is ultimately 

 necessary for them to gain access to a new host, and this is done in 

 one of two ways. There are produced certain resistant forms which 

 are protected by impermeable and resistant coverings or cysts which 

 enable the parasites to pass long periods outside the body, either on 

 the ground or in water, where they are carried about casually till they 

 are accidentally eaten by another host ; or special forms of the 

 parasite are developed, and these are taken up directly by some blood- 

 sucking invertebrate in which they survive and multiply till they are 

 finally transferred to another victim. The majority of microscopic 

 animal parasites adopt the first method of transmission, which may 

 be regarded as the more primitive one. Most, if not all, of the 

 intestinal protozoal parasites of man multiply in the intestine and 

 eventually produce forms protected by cysts. These escape from the 

 intestine in the dejecta, and are disseminated in various ways by 

 water and flies till they are accidentally eaten by some other human 

 beings. The parasite then comes out of its cyst and establishes itself 

 in the intestine. 



In the course of evolution the second type of transmission has 

 arisen. It is evident that if intestinal parasites limit themselves to 

 the intestine it is easy for the transmission forms in the encysted 

 condition to gain the lumen of the intestine and find their way to the 

 exterior ; but if the parasite changes its habitat, and instead of living 

 in the intestine, or the wall of the intestine, it selects the blood, it is 

 much more difficult for the transmission forms to find their way to 

 the intestine again. On the other hand, being in the blood they can 

 escape from the host in another manner — namely, by way of blood- 



at the posterior end of the body, and a single nucleus ; k, side view of 

 the flagellate ; l, encysted form, showing four nuclei. 



m-o Trichomonas hominis, which lives in the large intestine and sometimes 

 in the lower part of the small intestine. 



M, form with three flagella (Tritrichomonas) ; n, form with four flagella (Tetra- 

 trichomonas) ; o, form with five flagella (Pentatrichomonas). 

 There is a small cytostome or mouth, a single nucleus, an axial rod called 



the axostyle, which protrudes from the posterior end of the body, and another 



flagellum which runs along the border of an undulating membrane which is 



supported by a fibre. The encysted forms are not known. 



