184 



METAZOAN PHYLA 



another and to give rise to generation after generation. Rats may 

 become infected by eating infected rats or infected scraps of pork in 

 garbage. While pigs may eat an infected rat, they more commonly 

 become infected from garbage containing scraps of pork. It has been 

 shown that grain-fed pigs are less likely to be infected than garbage-fed 

 pigs. Man comes into this cycle of parasitism by eating inadequately 

 cooked pork and thus taking in the larvae, which pass the rest of their 

 life history within his body. The disease is very serious, heavy infections 

 being often fatal. 



The disease (trichinosis or trichiniasis) is difficult to diagnose, and 

 particularly so, since light infections produce mild symptoms. Most 



cases pass undiscovered. Recent examinations 

 of cadavers reveal that infection with this para- 

 site is very common throughout the United 

 States, probably 12 to 15 per cent of the people 

 containing at least a few larvae in their muscles. 

 About 4 to 5 per cent of garbage-fed hogs are 

 infected. Preventive measures include inspec- 

 tion of meat (federally inspected pork is made 

 safe by prolonged refrigeration), destruction of 

 infected meat, cooking of garbage or burning of 

 pork scraps, and, above all, the thorough cook- 

 ing of pork. Even infected meat, of course, is 

 perfectly safe when well cooked. 



215. Filaria. — The nematodes known as 



filariae include a number of species which affect 



both man and domestic animals, particularly 



in the tropics. The most injurious type is 



branches of water milfoil Wuchereria bancTofti (Cobbold), which is a 



{Myriophyiium) in which is threadworm living in the lymphatic system of 



man. The larvae of this parasite are carried 

 about in the blood, retreating to the center 

 of the body in the daytime but at night migrating to the peripheral 

 blood vessels in the skin. At times the adults seem to obstruct the pas- 

 sage of the lymph and in some cases may lead to an excessive growth 

 of tissue usually of the limbs resulting in a disease known as elephantiasis. 

 At night, when the larvae are active and are in the peripheral vessels, they 

 are sucked up by mosquitoes. They continue their development for a 

 time in the mosquito's body and when the mosquito bites another per- 

 son are transmitted to him. Elephantiasis is a very serious disease, 

 particularly in the South Sea Islands. 



216. Hairworms. — A worm which is popularly known as the horsehair 

 snake, and which is believed by many ignorant of zoology to be produced 

 in water from the hairs of horses, is by most authors placed in this 



Fig. 94. — Portions of two 



coiled a specimen of Gordius 

 Natural size. 



