DISPERSAL OF PARASITIC EGGS I99 



viviparous and its larvee are shed into the blood-stream or 

 lymphatics. The house-fly can therefore take no part in its 

 dissemination. The species in II {b) are, as might be expected, 

 but very occasional parasites of man. 



" In addition to the above-mentioned worms of which man 

 is the final or adult host, there are a few others for which man 

 figures as the intermediate host. By far the most important of 

 these is Ticnia echiiiococcus, a tape-worm of the dog, the larva 

 of which gives rise to hydatid disease. Man is also an inter- 

 mediate host of Tccnia solium and Ti'icJiiiiella spiralis. 



" It is evident from this short summary of the life-histories 

 that the only parasites with which it is possible that flies can 

 directly infect man are those in Section I, along with Tcenia 

 ecJunococcus and T. solium. In other cases and in the case of 

 T(EJiia solium also, the infection is conveyed to some other 

 animal, e.g. pig, ox, etc. The nature of the problem is therefore 

 not the same in everv instance." 



The nature and size of the eggs of parasitic worms. 



"These factors have an important bearing on the present 

 question, for it is obvious the eggs must bear some definite 

 proportion to the vehicle which carries them. In most cases 

 the eggs are ovoid in shape. Sometimes they are more elon- 

 gated, and become almost spindle-shaped. Frequently they are 

 nearly globular. Occasionally they are cuboid while a few other 

 shapes occur more rarely. Appendages are not uncommon. 

 They may take the form of slight roughnesses or small knobs 

 scattered irregularly over the surface of the shell. There may 

 be a small spike projecting from one end, or from the side. In 

 some cases one end of the shell tapers to a point and is pro- 

 longed as a spiral filament, which may be much longer than the 

 egg itself In certain other cases there may be button-like 

 projections from either end of the shell. 



" The shell may be of various degrees of thickness and hard- 

 ness. In some cases it is quite thin and transparent, in others 

 it is much thicker and opaque. In most cases it possesses a 

 considerable amount of flexibility, so that it can be compressed 



