PROPHYLAXIS 183 



plication taking place. The intermediate host according 

 to the species of the worm may be mammals, sometimes 

 man, arthropods, and in Bothriocephalus, fish. 



(3) Re-entrance into Alan. The larvae of Filaria ban- 

 crofti re-enter man, being discharged on to his skin by the 

 intermediate host, the mosquito ; and Filaria loa is trans- 

 ferred similarly by certain Chrysops. Probably other 

 filariae enter in a similar manner. 



The larvae of Ankylostomnm duodenale and Necator 

 americanns are capable of perforating the human skin, and 

 so re-enter man. The same probably takes place with 

 other nematode embryos, and it is possible that it is 

 through the skin that the Schistosoinnm hcematobium 

 enters man. 



Most of the worms with a simple development are 

 reintroduced into man by the mouth with food, rarely 

 with water. The eggs of the Ascaris lunibricoides and 

 Trichocephalm dispar, containing the living larvae, are 

 widely scattered after the motions with which they were 

 passed have disintegrated. These eggs in most instances 

 probably are carried with dust. When swallowed the 

 egg-capsule is dissolved off, and the embryos are set free 

 and soon become sexually mature. 



The filariform larvae of the free-living generation of 

 the Strongyloides are introduced by the mouth or penetrate 

 through the skin, and in the intestine develop into the 

 parasitic adult form. 



Mature ankylostome larvae are also introduced by the 

 mouth. These parasites therefore may enter either through 

 the skin or by the mouth, and probably the latter is in most 

 tropical countries of the greater importance. 



The cercariae of the Trematodes probably always enter 

 by the mouth, sometimes with water, but more frequently 

 with vegetables, as they are deposited on them by the 

 molluscs. Cestodes are introduced as cysticerci in food : 

 Beef (Cysticercus bovis), the scolices developing into Tcenia 

 saginata ; pork (Cysticercus cellulosce), developing into 

 T. soliiun. Of the other Taeniidae some are introduced 



