THE LIVER FLUKE I LIFE-HISTORY 



127 



larvae in a mollusc, although there are variations in detail and 



some species do not have an intermediate host. Another important 



example is Schistosoma {=Bilharzia), an important parasite of 



man of which various species are found 



in Africa, Asia, and America. The adults 



live in the veins, and are peculiar in being 



dioecious. The female lives for most of the 



time in a groove, formed by the rolling up 



of the male (Fig. 90), but she can leave 



this and does so to lay her eggs in the 



capillaries. It is the eggs which cause the 



disease schistosomiasis by penetrating the 



walls of the capillaries with their spines 



and by secreting an enzyme and causing 



haemorrhage. Eventually the eggs get into 



the bladder and leave the body in the 



urine. When this is diluted they hatch into 



miracidia which have a life of about 



thirty hours, during which they must 



infect a snail. Here two generations of 



sporocysts, and eventually cercariae, are 



formed, but there are no rediae. The 



cercariae are set free, and can penetrate 



any part of the skin of man. In countries where the parasite is 



common, any contact with fresh water is therefore dangerous, as 



a mere splash may bring the cercariae on to the skin. The 



parasite is very successful and in some areas is present in all the 



human population. 



Fig. go. — Schistosoma 

 hcBmatohinm. — From 

 Sedgwick. 



<J, male ; $, female ; S, sucker. 



CLASS CESTODA 



TMNIA 



TcBuia solium (Fig. 91), found in man, is an example of a tape- 

 worm. There is seldom more than one worm in the host (hence 

 the trivial name) but it may reach a length of twenty feet. It 

 lives in the intestine, in the wall of which the head or scolex is 

 usually buried. On the scolex are four lateral suckers, and a 

 crown of 22-32 hooks which are borne in two rows on a projection 

 or rostellum. Behind the scolex is a narrow neck, which is con- 

 tinually forming new sections of the body, called proglottides, 

 of which there may be as many as 1000. Each proglottis (there 



