60 



NA TURA L HIS TOR Y, 



in the tissues and cause the death of the unfortunate man from the enor- 

 mous number of worms which they will produce. 



The Ascarids are less dangerous to man, but two of the species are worthy 

 of mention. One of these is the round worm of the human intestine 

 (Ascaris lumbricoides) which reaches a length of several inches ; the other 

 is the "pin-worm " (Oxyuris cermicidaris), so distressing to children. It is 

 a small species which breeds in the human rectum, producing the most 

 intolerable itching. Yet small and delicate as it is, it will travel over the 

 sheets from one child to another, thus infecting a whole family. 



Worst of all the human parasites is the round worm, familiar by repu- 

 tation to all, — the dreaded trichina. For many years it has been known 

 that the eating of pork was dangerous and that not infrequently death 

 ensued from the sickness caused by it. Indeed, there is good reason to 

 believe that the Levitical prohibition was a sanitary measure based upon 

 a knowledge of this fact. But it was not until the last half of the present 

 century that the whole history of this sickness was known and the worm 

 Trichina spiralis recognized as the effective agent in trichinosis. The 

 history of the discovery is an interesting one, but it is surpassed in this 

 respect Iry the life history of the animal concerned. 



Swine are particularly liable to attacks of trichina, as of all other 

 parasites, on account of their omnivorous habits. They will eat almost 

 anything, and hence run a greater risk of swallowing the 

 eggs of parasitic animals than almost any other animal. 

 If the eggs of trichina be swallowed along with the food, 

 they hatch in the digestive tract of the pig, the young bore 

 their way through the walls of the intestine, and wander 

 to all parts of the body. After a wandering life of short 

 duration they settle down, preferring the muscular tissue as 

 their future home. Here they feed and grow, and event- 

 ually each encloses itself with a firm, white capsule just 

 large enough to be visible with the naked eye. Inside of 

 the capsule the worm lies coiled in a spiral, whence the 

 specific name of the animal, spiralis. Here it may remain, 

 maturing its eggs for an indefinite period, until at last the 

 fig. sa — Trichina pig is killed, and its flesh eaten by man. If the pork be 

 piece of human not cooked sufficiently to kill the encysted trichina, the di- 

 gestive juices quickly dissolve the capsule and set the worm 

 and its multitude of eggs free in the stomach and intestine. The eggs 

 soon hatch, and the embryos quickly bore their way through the intestine 

 to repeat in the human flesh their history in the body of the pig. 



It is while the myriads of worms are travelling through the body that 



