CHAPTER XXIV 



FREMATODA 



Metazoan parasites — Platyhelmia — Trematoda — Classification — Malacotylea 

 — Digenea — Prostomata— Paramphistomoidea — • Fascioloidea — • Schisto- 

 somi dee — References . 



SUBKINGDOM IL METAZOA. 



Definition.— Metazoa are free-living or parasitic, multicellular 

 animals, characterized by a physiological division of labour among 

 their cells. 



Remarks. — Tropical medicine is only concerned with parasitic 

 Metazoa, and chiefly with those which affect man. Parasitic Metazoa 

 may be ectoparasites- — -as, for example, many species of the Insecta — • 

 cr endoparasites— as, for example, many worms. The ectoparasites 

 can cause disease by introducing toxins, protozoa, or bacteria into 

 the tissues, and in this way they are of the utmost importance as 

 the spreaders of disease, for, as is generally recognized, a disease 

 is very often limited by the special oecological conditions of the 

 animal which spreads its germ. Many of the ectoparasites, such 

 as mosquitoes, are temporary ; or, like ticks, periodic ; while others, 

 like lice, are permanent parasites. 



With regard to the endoparasites, their ill effects on the host 

 depend upon many factors which have been recently studied in 

 considerable detail. 



The effects of metazoan parasites on their hosts depend upon the 

 species of the parasites, their condition, the number present, their 

 presence in certain organs, bacterial infection, their migration in' 

 the body, the loss to the host in feeding them, the damage caused 

 by their toxins, and the condition of the host. 



1. The Species of Parasite. — Filaria may exist in a host without 

 apparent ill effect, but Ai cylostoma will produce anaemia, oedema, 

 and perhaps death. 



2. The Condition of the Parasite.— The dead eggs of Filaria are 

 believed to block the lymph channels, and give rise to swelling and 

 rupture of these vessels, thus bringing about lymphangitis and 

 elephantiasis, while the living larvae apparently do no harm. 



3. The Number of Parasites.— A few Ascarides may cause no 

 symptoms, while a large number may lead to serious disease. 



4. The Organ Affected.^ — -Ascarides in the alimentary canal are not 

 nearly so virulent as in the liver, while Paragonimus ringeri 

 in the lungs will cause a disease somewhat resembling phthisis. 



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