Phyla Platyhelminthes and Nemertina 415 



and the sporocysts and rediae develop. The cercariae swim about 

 for a very short time (one to three hours) and encyst upon some 

 aquatic plant. Infections occur when uncooked water nuts or water 

 chestnuts are eaten. At times there may be thousands of cysts upon a 

 single nut. The use of night soil for the fertilization of the ponds 

 where these plants are grown ensures the continuance of this interest- 

 ing parasite. 



Actually the most important group of flukes which parasitize human 

 beings is that of the schistosomes or hlood flukes. These differ markedly 

 from the other flukes inasmuch as the sexes are separate. The male is in 

 some species about 8 to 16 mm. long and appears cylindrical. Actually 

 it is a flattened animal, but the sides of the body are folded toward 

 the midline, forming a protected groove. The slender female is carried 

 in this groove. In some species, the female remains permanently in 

 this position, in others she makes periodic excursions for the deposition 

 of eggs. The worms usually live in the mesenteric blood vessels where 

 the food supply is particularly abundant. The female, in most cases, 

 migrates to as small a blood vessel as she can enter. Here she deposits 

 her spined eggs. These eggs work their way into the wall of the 

 intestine or bladder. They are then expelled with the urine or feces. 

 When diluted with water, the eggs quickly hatch into miracidia which 

 burrow into suitable snail hosts. Here they elongate and become sporo- 

 cysts. These produce daughter sporocysts which repeatedly develop 

 and expel forked-tailed cercariae. Because the sporocysts do not die, 

 but continue to produce new cercariae, tremendous numbers may be de- 

 veloped from a single miracidium. These cercariae escape into the water 

 and burrow into the skin of individuals who are swimming in infected 

 waters. The larvae work their way into the blood system, pass through 

 the heart and lungs, finally appearing in the liver. Here they feed on 

 portal blood, develop into adult individuals, and finally enter the 

 mesenteric veins. 



The diseased condition resulting from an infection is known as 

 schistosomiasis, or at times as hilharsiasis. 



Three species which infect human beings are important, and these 

 occur in Africa, the Near E^st, the Orient, parts of southern Europe, 

 and the New World tropics. In infected individuals, they do consider- 

 able damage and are often fatal. In Egypt, for example, the percentage 

 of infection among the poorer groups may run as high as 85 per cent 

 and in some areas directly account for 10 per cent of deaths. 



