VIII] LIFE-HISTORY 131 



perpetual parasite. This Nematode inhabits the intestine of its 

 host (Pig, Man, etc.) where it lays its eggs. Prom these eggs 

 larvae hatch out which bore through the walls of the intestine and 

 get into the circulation, by which they are carried all over the body. 

 They encyst themselves in 

 the muscles (Fig. 56) and 

 do not develop further un- 

 less the flesh of their host is 

 eaten by another animal, in 

 whose intestine they become 

 mature and then thecycleof 



development recommences. FlG - 56 - Trichina spiralis, encysted 



fm . T , , . , , amongst muscular fibres. Highly mag- 



Their natural host is the nified . After Leuckart. 



Rat ; the Pig is a secondary 



host and being a gross feeder no doubt often devours rats and the 

 remains of its own species, and thus the parasite is propagated. 

 Trichina can however live perfectly well in Man, as the prevalence 

 of the disease trichinosis testifies. This disease is contracted by 

 eating insufficiently cooked pork infested by the encysted larvae of 

 Trichina. These become mature in the human intestine, and give 

 rise to a second generation which cause severe and occasionally fatal 

 symptoms by boring through the intestinal wall. 



Most Nematodes pass the earlier part of their existence in 

 damp earth, during which they are known as Rhabditis larvae and 

 bear a strong resemblance to some of the free-living forms. Tylen- 

 chus tritici forms the so-called Ear-cockles in wheat; these are 

 brown galls replacing the wheat grains and filled with encysted 

 Nematodes. If the grains are beaten to the earth by rain the 

 worms escape from the cysts and climb up the wheat stalks, where, 

 after a generation which live in the flower, they again enter the 

 grains. In Spha&rularia bombi the males and females live together 

 in damp earth, but the fertilised female enters the body of a Bee 

 and here develops into a great sac filled with eggs. In Syngamus 

 trachealis the eggs are laid in damp earth, and here develop into 

 larvae, which are swallowed by poultry and develop in their wind- 

 pipes into the sexual form, causing the disease called gapes, which is 

 often fatal. Filaria sanguinis-hominis lives in the human lymph- 

 atic glands; the embryos escape into the blood, whence they are 

 taken up by the Mosquito in whose body they develop. When the 

 Mosquito bites they make their way again into the blood of Man. 

 They are the cause of a peculiar form of malaria known as "filariasis." 



92 



