plunged into cold water, as in laundering or in dip- 

 ping up water from wells or village ponds. Tlie tiny 

 worms swim about until they perish or are swallowed 

 by a second host, a species of Cyclops, a tiny crus- 

 tacean. When drinking water is dipped up (often by 

 the same individuals who infect the water by stand- 

 ing in it as they lower their buckets) it contains in- 

 fected crustaceans that harbor larval worms. Rede- 

 signing wells and filtering water could eliminate this 

 disease, but in India religious traditions that surround 

 the ways of obtaining and using water have made 

 change slow. 



More than fifty species of roundworms parasitize 

 man occasionally, but only about a dozen are impor- 

 tant human parasites. Five have already been men- 

 tioned. Some others are the subtropical and tropical 

 filarial worms that cause elephantiasis; the African 

 eye worm, Loa loa; and the world-wide whipworm, 

 Trichuris trichiura, which usually lives in the large 

 intestine, causing symptoms ranging from abdom- 

 inal pain to severe emaciation and prolapse of the 

 rectum. 



The one roundworm most likely to have parasit- 

 ized readers of this book is the pinworm (seatworm 

 or threadworm) Enterobhis venniciilaiis, found all 

 over the world, but rare in the tropics. It flourishes 

 in Europe, where even the cleanly Dutch children 

 are said to be 100 per cent infected, and in North 

 America, where sample surveys show that 30 to 60 

 per cent of white children in Canadian and Ameri- 

 can cities have pinworms. Negroes are less suscep- 

 tible, and in Washington, D.C., Negro children have 

 an incidence of only 1 6 per cent, compared with 40 

 per cent for white children. These are little white 

 worms, the females up to Vi of an inch long, that 

 live in the cecum, appendix, and adjacent parts of 

 the large intestine. When the females are full of eggs 

 they migrate to the rectum and lay their eggs around 

 the anal opening. Their movements cause intense 

 itching, often sleeplessness and nervousness. Scratch- 

 ing of the anus, and liberation of the eggs into the 

 air and onto sheets and clothing, spreads the eggs 

 about so effectively that in some households and in- 

 stitutions eggs can be taken from almost any surface 

 or object. It is easy to imagine how the eggs reach the 

 mouths of adults, but more especially of children, in 

 such places. For this worm, treatment is easier than 

 prevention. 



The Rotifers 



{Phylum Rotifera) 



One of the most fascinating, and busiest, of sights 

 is a drop of pond water magnified to reveal a field of 

 feeding, crawling, and swimming rotifers. These in- 

 tensely animated microscopic creatures occur in a 



great variety of fantastic shapes and handsome sur- 

 face sculpturings. Their greatest attractions, how- 

 ever, are their incessant external activity and a trans- 

 parency that displays the lively inside workings as 

 might a glass model. 



After the bewilderment induced by a first glimpse 

 of a vivacious rotifer, attention centers on an eye- 

 catching piece of gadgetry at the front end, the co- 

 rona or "crown," used for both feeding and swim- 

 ming. It includes the mouth and the more or less ex- 

 panded area of delicate ciliated skin surrounding or 

 close to the mouth. In the hunting rotifers, which go 

 forth in search of food, swimming or gliding through 

 food-laden water, the corona is external and often 

 convex. In many species which live permanently 

 fixed by a long stalk, or in those which attach tem- 

 porarily while feeding, the coronal lobes may be 

 protruded from the mouth during feeding, then re- 

 tracted. Some of the large and beautiful stationary 

 rotifers have a lobed and funnel-shaped corona with 

 long bristles that prevent escape of the prey when 

 the lobes of the funnel close down on some small ani- 

 mal that happens to enter. 



The most familiar rotifers of fresh waters are the 

 bdelloid ("leechlike") rotifers, elongate little ani- 

 mals that creep in leechlike or inch-worm fashion on 

 the bottom or on plants. Bdelloids typically have a 

 corona consisting of two elevated disks, and these 

 propel the animals on brief excursions through the 

 water. The large fused cilia that fringe the two coro- 

 nal disks beat in such a way as to create the illusion 

 of two rotating cogged wheels. These were the first 

 rotifers discovered by the early microscopists, so that 

 long before the illusory matter was finally cleared 

 up, all the microscopic creatures with expanded 

 crowns of cilia at the front end had been named 

 "wheel animalcules." The formal name of the phy- 

 lum, Rotifera, means "wheel-bearers." 



Rotifer shapes may be wormlike, as in bdelloid 

 rotifers; flower-like, as in the sessile forms that have 

 great expanded coronas; or rotund, as in the rotifers 

 that float freely in open water. The common fresh- 

 water bdelloid rotifer, Philodina, has an elongate 

 body distinguishable into a corona-bearing head, a 

 central region or trunk, and a tapering foot region. 

 At the end of the foot are two pointed projections 

 called "toes," and from each of these open cement 

 glands that secrete a sticky substance by which the 

 animal anchors temporarily while feeding. The toes 

 are also of use in creeping about, as the flexible body 

 alternately lengthens and takes hold by the front end, 

 then contracts and fastens by the toes. The whole 

 body is enclosed in a flexible cuticle which is folded 

 into sections that telescope into each other when the 

 animal contracts. In some rotifers the cuticle of the 

 trunk region is hardened into an armored case or 

 lorica, either smooth or ornamented with grooves or 



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