436 The Animal Kingdom 



and considerable dooryard pollution. Through this country, south- 

 west Louisiana and the southern Appalachian areas, are foci of in- 

 fection. The adult worms (female 30 to 50 mm. long) have the an- 

 terior two-thirds thin and threadlike. In the living worm, this portion 

 whips about. Often the worm threads this slender portion through the 

 wall of the large intestine. The worm may live for years, and the in- 

 fection builds up slowly; thus it is often difficult to diagnose. The 

 usual symptoms are gastrointestinal disturbances. Like any worms 

 that live in the large intestine, it is often difficult to kill them. They 

 usually live too high to be reached by enemas, and most drugs are in- 

 effective when they have travelled that far through the digestive tract. 

 The life cycle is direct, the eggs developing slowly in the moist soil and 

 being ingested by the host. They hatch near the caecum. 



Elephantiasis. — L'nlike the other nematode parasites of man men- 

 tioned above, IVuchereria bancrofti needs an intermediate host to 

 complete its life cycle. W. bancrofti belongs to the group known col- 

 lectively as the filarial zi'orms. This worm is known from most of the 

 tropical regions of the world and in some areas infects more than 50 

 per cent of the inhabitants. 



The adult worms live in the lymph glands and lymph passageways. 

 The females are from 65 to 100 mm. in length and very slender; the 

 males are smaller. These adult females produce numerous young, the 

 microfilariae. Further development depends on their being picked up 

 by the proper mosquito. Much investigation needs to be done on the 

 movements of these microfilariae in the blood of the human host. It 

 is known that their movements seemingly are dependent upon the activ- 

 ity of the host. In many areas where the microfilariae are carried by 

 night-flying mosquitoes, they come to the surface blood vessels at 

 night ; those carried by day-flying mosquitoes come to the surface ves- 

 sels at all hours. If the microfilariae are picked up by a mosquito the 

 further development proceeds very rapidly. They move to the walls 

 of the mosquito's stomach, penetrate them, and reach the thoracic 

 muscles within twenty-four hours. Growth continues for several weeks 

 until the worm attains a size of about 1.5 mm. They then crawl down 

 the proboscis of the mosquito. When the mosquito bites a man, the 

 larvae crawl on the surface of the skin and enter through the hole left 

 by the mosquito. 



Since only a very small number of worms enter at a single time, 

 it usually takes repeated inoculations over a series of years before 



