146 ANIMAL LIFE AND HUMAN PROGRESS 



host, in which it remains encysted until this vehicle-host is 

 eaten by man as food. For each species, however, the second 

 host, i.e. that in which the cercaria or " infective stage " en- 

 cysts, is not the same. The lung-fluke, after passing through 

 Melania, utilises certain fresh- water crahs as a means whereby 

 to reach man in encysted form. The two other flukes encyst 

 in a variety of fresh-water fishes which have this in common — 

 that they are usually consumed as human food. 



Taking the zoological facts concerniag these three flukes 

 together, two methods of prevention obviously present them- 

 selves. Either the use of fresh-water crabs and fish must be 

 restricted and these animals only allowed as food after sterihsa- 

 tion by heat etc. , or some attempt must be made to eradicate the 

 molluscan intermediary which is essential to the development 

 of all three parasites. The latter plan seems the simpler and the 

 more feasible. In Korea some prehminary experiments have 

 already been undertaken to determine whether, by the collec- 

 tion of Melania, its local incidence can be kept under control. 



The Bilharzia worms ^ Uve in the blood-vessels and give rise 

 to eggs which, after migrating through the tissues, erode their 

 way through the walls of the bladder and gut, causing symp- 

 toms of haematuria and dysentery. The eggs of these flukes 

 are passed in the urine and excrement. It has recently been 

 shown that they hatch ia water and give rise to cihated bodies 

 which attack certain species of fresh- water snails. The meta- 

 morphosis which then takes place follows along lines sinoilar 

 to those characteristic of flukes generally, and the final stage 

 leaves the snail as a free-swimming form. This cercarial or 

 " infective " stage, however, does not encyst in some food- 

 animal, as described above for the other fluke parasites of 

 man. It has been proved, by experiments upon rats, guinea- 

 pigs and monkeys, that in the case of the Bilharzia worms the 

 free-swimming cercaria can actively force its way into the 

 human body through the skin and that a second intermediary 

 or vehicle-host is therefore unnecessary. 



^ There are three species, viz. Schistosoma haematobium, Schistosoma 

 mansoni and Schistosoma japonicum. 



