ANIMAL PARASITISM 447 



fever that many cases are diagnosed as typhoid by good physicians. 

 Nine or ten days after the beginning of infection, when the larvae 

 are migrating to the muscles, there are severe muscular pains and 

 aches (sometimes diagnosed as rheumatism), and the inflammation of 

 the muscles used in breathing, chewing, etc., may interfere with these 

 functions. As the parasites become encysted, about six weeks after 

 infection, pains become worse and swelling of the infected parts 

 occurs, accompanied by anemia and skin eruptions. If the victim 

 survives this period he usually recovers, as the parasites are now 

 walled off by cysts of connective tissue formed by the host ; later 

 calcium carbonate is deposited in these cysts, walling off the parasites 

 so completely that they die, but the calcified cysts remain as hard 

 grains in the muscles and may cause some rheumatic pains for years. 

 Until the worms are completely walled off fever is caused by poison- 

 ous substances produced by the larval worms. Recent studies of the 

 cadavers used in medical schools have revealed that about 20 per 

 cent of the American population probably have cases of trichinosis 

 in some degree at some time during life, since about this proportion 

 of the cadavers had trichina cysts in the muscles. 



All danger of trichinosis can be avoided by cooking pork thor- 

 oughly before eating it, as the larvae are killed by a temperature of 

 55° C. (131° F.). Investigations of the United States Bureau of 

 Animal Industry, in which the author assisted, indicate that pre- 

 pared sausages seldom contain living worms, most of them being 

 killed by the salts and seasoning or by long-continued cold storage ; 

 the greatest danger is from fresh pork. Contrary to popular im- 

 pression. Federal inspection does not guard against trichina, as there 

 is no effective way to inspect meat for trichina on the large scale that 

 would be necessary. 



Trematodes. — Schistosoma haematobium, a Human Blood Fluke. — 

 The blood flukes (Family Schistosomatidae) are distinguished from all 

 other trematodes by having separate sexes. The male has a thick body 

 with the lateral edges bent ventrally, thus forming a long groove on 

 the ventral surface, the gynecophoric canal. The female is long and 

 slender, almost threadlike in some species; when an adult male and 

 female happen to come in contact, the male folds his body around 

 the female so that she is held fast in the gynecophoric canal, and the 

 pair begin copulation. After once becoming paired they remain in 

 copula during the rest of their lives. Three species of the genus 



