THE TUBE-WITHIN-A-TUBE BODY PLAN 



181 



in that the mouth is provided with teeth so 

 it can cHng to the soft mucosal lining of the 

 intestine from which it withdraws its food, 

 blood (Fig. 10-4). Fertile eggs pass out 

 with the feces of the host and are deposited 

 on the ground where they hatch into larval 

 worms. After a brief growth period the lar- 

 vae are ready for their next host. They gain 

 entrance by holding on to the foot or any 

 other part of the body of the host, boring 

 in, and finally getting into the blood stream. 

 They then follow the same path described 

 for ascaris, eventually reaching the intes- 

 tine. 



Preventive measures are so simple that 

 one wonders why there are any cases of 

 hookworm at all. Wearing shoes prevents 

 the worms from getting into the host; 

 proper methods of disposing of human ex- 

 creta would also stop the infection very 

 swiftly. Both of these methods have been 

 tried with reasonable success but the inci- 

 dence of hookworm disease is still much too 

 high in this country. Perhaps the most im- 

 portant factor is poverty; if all people were 

 gainfully employed and had an adequate ed- 

 ucation this disease would be completely 

 eradicated. World-wide measures could 

 stamp it out altogether, but such suggestions 

 are only wishful thinking at the present 

 time. 



Another roundworm parasite that is of 

 grave importance to man is trichina ( Trichi- 

 nella spiralis), a worm whose normal hosts 

 are the pig and rat, although it has been 

 found in other vertebrates as well. Man is 

 an accidental host and is therefore perhaps 

 even more severely affected by the parasite. 

 The life cycle of trichinella varies somewhat 

 from the two preceding examples of round- 

 worm parasites (Fig. 10-5). The common 

 source of human infection is through the 

 muscle of the pig, which harbors trichina in 

 its infective stage, small cysts containing 

 larvae (Fig. 10-6). If these are eaten, 

 uncooked, the tiny worms ( 1 mm. long ) 

 emerge in the intestine where they mature 

 and copulate. The very tiny worms depos- 



F!g. 10-6. Larval Trichinella cysts in the muscles of a rat. 



ited by the female bore through the in- 

 testinal wall into the blood stream and dis- 

 tribute themselves through the muscles of 

 the body, attacking particularly those of the 

 tongue, eyes, diaphragm, and ribs. It is this 

 migratory period that is dangerous because, 

 in addition to the mechanical injury that 

 millions of worms can inflict, there is also 

 the likelihood of bacterial infections. The 

 disease at this stage is characterized by 

 high fever, intense muscular pains, and 

 frequently partial paralysis. A sufficient 

 amount of infected meat can cause death at 

 this time, but if the infection has been 

 light enough not to cause permanent dam- 

 age to nervous and muscular tissue, the 

 symptoms will subside and the person re- 

 cover. Infections are far more common than 

 records indicate. For example, a large Mid- 

 western hospital reported that 27 per cent 

 of its autopsies showed positive trichinosis, 

 although none of the deaths was directly 

 caused by that disease. 



Preventive measures are even simpler 

 than for hookworm: merely cook pork. 

 There is no treatment for the disease once 



