196 DISPERSAL OF PARASITIC EGGS 



but also the larvje of Necator americanns. Calandruccio (1906) 

 observed a number of flies, which had fed on material containing 

 the eggs of Hynienolepis nana, deposit faeces containing the eggs 

 on sugar. A girl who had eaten some of this sugar was found 

 twenty-seven days later to be infected with the tape-worm. 

 Other sources of infection were carefully excluded. Leon (1908) 

 fed flies on honey mixed with the eggs of DibotJiriocephalns latns, 

 and found the eggs in their faeces. 



Nicoll (1911) has carried out by far the most extensive in- 

 vestigations on this subject, experimenting with ten ditTerent 

 species of parasites infesting men or animals. 



Nature and life-history of parasitic worms. 



" In order to indicate the precise relationship which flies and 

 other insects may bear to the dissemination of parasitic v/orms, 

 it may be advisable here to recapitulate briefly the principal 

 facts which are known concerning the mode of life and means 

 of transmission of these worms. They belong to three principal 

 classes, namely Trematodes or flukes, Cestodes or tape-worms 

 and Nematodes or round-worms. These worms in their adult 

 state live for the most part in the alimentary canal of vertebrate 

 animals, Practically speaking all tape-worms live in the intestine, 

 the great majority of round-worms and flukes live in the intestine, 

 stomach or cesophagus, but certain varieties live in the lungs, the 

 liver, the kidneys, the bladder, the blood and lymphatic vessels. 

 In whatever situation they live, however, they all possess the 

 common characteristic of being unable to multiply without some 

 intermediate external influence. They all produce a large number 

 (in many cases an enormous number) of eggs, but before the 

 latter can grow into adult worms, they must pass part of 

 their life outside their host under certain definite conditions, 

 which vary according to the particular kind of worm. In most 

 cases the eggs are conveyed out of the body in the faeces, in 

 some cases in the urine or expectoration. Some tape-worms 

 throw off a segment containing eggs, which may pass out inde- 

 pendently of the faeces. The chief exceptions to this method 

 are the Filaria worms and their allies. With these, however, the 



