104 PROTECTION OF PLANTS, 1916-17 



Another large tapeworm of man is Taenia solium. The chief interme- 

 diate host of this worm is the hog. Pork or beef infested with cysts of tape- 

 worms is spoken of as "measly." 



These must suffice for illustrations of life histories of the Cestoda of 

 man and the higher animals. It will be seen that in all cases the eggs de- 

 veloped in the primary host are scattered with its excrement and that they 

 must then be sAvallowed by another species of animal — the secondary host — 

 in which they undergo development before they are capable of reinfecting 

 the primary host. 



With most of the Nematoda, or threadworms, affecting man this is not 

 the case. Before taking up other illustrations however, we shall consider 

 one of the most important nematode parasites which, like the tapeworms, 

 must pass through tw^o hosts in completing its development. This is the 

 trichina worm (Trichinella spiralis), the cause of the serious and often fatal 

 disease of man known as trichinosis. The larval worms live coiled up in lem- 

 on-shaped capsules in the muscle of man, pigs, rats, and various other 

 animals. When man feeds upon infected pork the larvae are liberated and 

 develop as mature male and female forms in the intestine of their new host. 

 After fertilization the females produce enormous numbers of living young, 

 which migrate to the muscular tissues and there in turn become encapsuled. 

 The natural reservoirs of the parasite are probably rats. These become 

 infected through eating the offal and scraps of infected meat about slaughter 

 houses, and, being cannibalistic, they afford an easy means for the develop- 

 ment and spread of the trichinae. I have found as high as sixty per cent 

 of the rats taken at a local slaughter house to be infested. 



Probably the most common of the parasitic worms of man, the world 

 over, is the so-called eel-worm, Ascaris lumbricoides . It is also, with very 

 rare exceptions^ the largest, the mature males measuring about eight inches 

 and the females up to fifteen inches in length. In their fresh condition they 

 somewhat resemble an enormous earthworm. The eggs leave the body of the 

 host with the feces, and scattered by wind and rain, they develop an embryo 

 which may remain dormant for many months, or even for years. Sooner 

 or later some of them may be swallowed by man, in water, salads and other 

 uncooked vegetables, or from dirty fingers. Then in five or six weeks the 

 embryos develop into adult worms, and the eggs are again scattered broad- 

 cast. 



A common nematode worm of man is the "pinworm," Oxyuris ver- 

 micularis. It is this worm that is usually meant in Speaking of "worms" in 

 children, though statistics show that it is less abundant in this country than 

 has been commonly assumed. The males are about one-fifth of an inch in 



