. Natural history. Zoology. THREAD- WORMS. 681 space to notice is of special interest, not only on account of its being one of the most dangerous of parasitic worms to mankind, but also because it spends the whole of its existence within the bodies of two hosts, carrying the parasitic habit to a greater extreme than is met with elsewhere in the group. This is the worm called Trichina spiralis, which gives rise to the dreaded disease trichinosis. In the adult or sexually mature stage this worm lives in the intestine of various carnivorous mammals, man being amongst the number. But the minute yo
. Natural history. Zoology. THREAD- WORMS. 681 space to notice is of special interest, not only on account of its being one of the most dangerous of parasitic worms to mankind, but also because it spends the whole of its existence within the bodies of two hosts, carrying the parasitic habit to a greater extreme than is met with elsewhere in the group. This is the worm called Trichina spiralis, which gives rise to the dreaded disease trichinosis. In the adult or sexually mature stage this worm lives in the intestine of various carnivorous mammals, man being amongst the number. But the minute young, which are born alive, instead of making their way to the outer world in connection with excrementitious matter, bore through the wall of their host's alimentary canal and entering the blood-stream are carried by the circulation to various parts of the body, ultimately coming to a standstill in the muscles. Here the little creatures begin to grow and form for themselves a protective capsule or cyst between the fibres of the muscular tissue. Meat infested with these encysted worms is said to be " trichinised " (Fig. 8, a). In this state the Trichina remain without attaining maturity until the meat in which they have taken up their abode is eaten by another animal, whereupon the action of the digestive juices of the new host, dissolving away the muscular fibres and the capsule, set free the worms, which in their new surroundings quickly become adult, and each female gives birth to thousands of new individuals. Man as a rule contracts "trichinosis" through eating "trichinised" pork in an underdone state. But, happily, since the discovery of the source of infection some fifty years ago, and the introduction of stringent laws against the sale of unhealthy pork, the disease has become far less common than in former times. Usually classified with the thread-worms, though differing from them in the absence of mouth and alimentary canal, and in other an
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