448 ESSENTIALS OK ZOOLO(JY 



Schistosomurn are human parasites: Schistoso7na mansoni, found in 

 Africa and the tropical parts of the New World, 8. japonicum of 

 Japan and China, and S. haematobium, the Egyptian blood fluke 

 found in north Africa and southwestern Asia. Schistosoma haema- 

 tobium is a parasite in the large blood vessels of the rectum and 

 urinary bladder. The female lays an enormous number of eggs, 

 which collect in the capillaries of the bladder and intestinal walls 

 and block the flow of blood, causing the infected parts to become 

 swollen and ulcerlike. Eventually the eggs are released into the 

 lumen of the rectum or bladder and pass out of the host's body in 

 the feces and urine, along with considerable quantities of blood from 

 the torn tissues. Aside from the pain caused by the egg-filled swell- 

 ings, the loss of blood is the most serious effect of blood-fluke infection. 

 Infected individuals are kept in a run-down anemic condition. The 

 parasite is a serious public health problem in Egypt, where 80 per 

 cent of the population of the Delta region is infected. 



The egg must fall into water for the embryo to develop into a 

 ciliated larva, the miracidium. When the miracidium is fully de- 

 veloped, the cap or operculum on the end of the egg shell is pushed 

 open and the liberated miracidium begins to swim around in the 

 water. Certain species of snails seem to emit a chemical which at- 

 tracts schistosome miracidia; if a snail of the right species is in the 

 vicinity, the miracidium swims to it and enters its body. If no suitable 

 snail is available, the miracidium dies after swimming a few hours. 

 After penetration into a snail the miracidium loses its cilia and de- 

 velops into a long sausage-shaped sac, the sporocyst, without any 

 recognizable organs. Germ cells within the sporocyst develop into a 

 number of young sporocysts, which escape from the mother sporocyst 

 into the tissues of the snail, grow to full size, and then in turn give 

 birth to a new generation of sporocysts, or, under some conditions, 

 these sporocysts may give rise to a different kind of larva, the 

 cercaria, which has a. long, forked, muscular tail, a pair of eyespots, 

 two suckers (one anterior and one midventral), and a rudimentary 

 digestive system. When fully developed, the cercaria forces its way 

 out of the snail and begins to swim through the water. If unsuccess- 

 ful in finding a host, the cercaria dies in a few hours, but if it comes 

 in contact with the skin of a man, or any other mammal, the cercaria 

 enters the skin, with the aid of glands in the head region which seem 

 to be used in digesting or destroying the skin tissues. After pene- 

 trating the skin the cercaria soon finds its way into a blood vessel 



