548 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



position at the roots of the grass, and so on, and in 

 time either die down, or are eaten by a sheep. When 

 the latter misfortune happens, they pass into the 

 stomach, and so to the gall ducts and liver, to grow 

 up afresh into the likeness of 

 the liver-fluke from which they 

 started. 



Histories not unlike those of 

 these two divisions of the Platy- 

 helminthes are presented by the 

 round worms or Nematoliel- 

 mintlies, and by the Echino- 

 rliyiiclii. The thread - worm 

 of the human blood (Filaria 

 sanguinis hominis), which appears 

 to be the cause of chyluria and 

 of some other diseases in the 

 countries of the Eastern Old 

 World, has been found to have 

 an intermediate host in the 

 mosquito, from whom it passes 

 into water ; when this water is 

 drunk the young return to the 

 human intestine. Dracunculus 

 medinensis lives in its adult con- 

 dition in the subcutaneous tissue 

 of the human leg and foot : its 

 larval stages being passed, as it 

 seems, in a fresh- water crustacean. 

 Trichina is an example of a form 

 which appears to have had its history modified ; in 

 societies that may be called cannibal (e.g. rats) no in- 

 termediate host would appear to be necessary ; in 

 the case of civilised man, the adult worms are obtained 

 from the flesh of incompletely-cooked pigs. 



Fig. 229. Cefcaria of D. 

 hepaticum. (After 

 Thomas.) 



