CHAPTER V 



Free-living turbellarian, tapeworm and flnke 



The Flatworms 



{P/iylnfft PlatyhdmhitJics\ 



o. 



NE may easily pass a lifetime without ever see- 

 ing a fiatworm. The smallest ones are microscopic, 

 and the largest ones, the ribbon-like tapeworms that 

 may grow up to 50 feet or more in length, develop 

 and pass their adult lives safely hidden within the 

 bodies of their human or other vertebrate hosts. They 

 are seen only when they die or are rudely removed 

 by medical treatment. Best known, perhaps, are the 

 Vi-inch planarians used in classroom study and in 

 zoological research. In nature these live unobtru- 

 sively in springs, streams, and ponds, crawling about 

 on the vegetation or under stones. After gathering 

 wild watercress one may have to rinse out little pla- 

 narians. On marine shores the oval and leaflike poly- 

 clads, some of them 2 inches long and colorful or 

 beautifully striped, may be seen by turning over boul- 

 ders or peering into sheltered rock overhangs when 

 the tide is out. Tantalizingly hard to find are the land 

 planarians of moist temperate woods; by their noc- 

 turnal and retiring habits they elude even the serious 

 students of flatworms. The occasional land planar- 

 ian that turns up in temperate gardens or in green- 

 houses is usually an import from tropical lands, 

 brought in with exotic plants. 



Yet for all this, free-living flatworms are abundant 

 and widespread; while the importance of the para- 

 sitic kinds in human history and in modern economic 

 and political problems can hardly be exaggerated. 

 One kind of parasitic flatworm, the blood fluke Schis- 

 tosoma, lives in the blood vessels of more than a hun- 

 dred million people. In World War II it helped to de- 



termine the outcome of many military actions in the 

 South Pacific. Parasitic flatworms are still very much 

 a part of the African and Asian pattern of disease, 

 low productivity, and poverty. If the pattern is to be 

 broken, the flatworm parasites that flourish espe- 

 cially — though by no means entirely — in tropical 

 countries will have to be more widely understood and 

 coped with. Some things that we think of as progress 

 in many countries, such as the building of dams to 

 supply irrigation canals, tend to increase the spread 

 of blood flukes. To understand why, one needs to 

 read the brief account that will be given of the life 

 cycle of such flukes. In temperate latitudes Euro- 

 peans and Americans, their pets, and their livestock, 

 are still subject to infestation with flukes and tape- 

 worms, though some of these have been brought un- 

 der control. Many people who think of such para- 

 sites as occurring only under very unsanitary condi- 

 tions have had "swimmers' itch" caused by the larval 

 flukes that develop in numerous lovely lakes favored 

 as summer resorts. 



There are three classes of flatworms, roughly esti- 

 mated to include almost nine thousand known spe- 

 cies, only a fraction of the number that actually exist. 

 The first consists almost entirely of free-living little 

 worms. The other two classes, the flukes and the 

 tapeworms, are exclusively parasitic and far more 

 numerous. These are not attractive animals to the 

 average layman, and when Aristotle became fasci- 

 nated by the various worms that live in man, he felt 

 obliged to justify his curiosity in these words: "In all 



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