. Biology . ands ofembryos, each capable of giving rise to a new tapeworm. Butthese are deposited with the faeces, and beforethey can develop into a new Taenia mustundergo partial development in the pig. Inone way or other, they find their way intothe food of a pig; the embryos are liberatedby action of the pigs digestive fluids, andwhen liberated make their way through thewalls of the digestive tract into the musclesof the pig. Here their development is ar-rested, and, as cysticercids or bladder-worms,they give rise to what is called measly pork eaten in an uncooked state is a spira
. Biology . ands ofembryos, each capable of giving rise to a new tapeworm. Butthese are deposited with the faeces, and beforethey can develop into a new Taenia mustundergo partial development in the pig. Inone way or other, they find their way intothe food of a pig; the embryos are liberatedby action of the pigs digestive fluids, andwhen liberated make their way through thewalls of the digestive tract into the musclesof the pig. Here their development is ar-rested, and, as cysticercids or bladder-worms,they give rise to what is called measly pork eaten in an uncooked state is a spiralis, encysted in source of human infection. The bladder-muscle tissue. (From r j^1 J J. X xi- Hertwig after Boas.) worms are freed m the digestive tract, be-come attached as scolecids to the liningepithelium, and begin to bud out proglottids. Here, there is a very characteristic physiological adaptation,in which the difficulties of maintaining the species are balancedby the enormous number of embryos Trichina SYMBIOSIS, COMMENSALISM, PARASITISM 193 In a similar way, thousands of species of animals becomeadapted to a parasitic life in different types of host. Further-more, there are different grades of parasitism; some are obliga-tory parasites, requiring a particular host and a particular organ,very often, of that host. Others are facultative parasites, notabsolutely dependent on a given host, but capable of living insuch a host if chance brings them there. Thus the round worm,Trichina, is an obligatory parasite of man, and a facultativeparasite of domesticated animals, including the pig. Theembryos are eaten with infected meat; liberated in the humanintestine, they penetrate the walls of the digestive tract andmultiply in the body cavity, ultimately penetrating musclebundles where, in the muscle cells, they finally encyst. If theunfortunate victim does not die from trichinosis before suchencystment occurs, recovery is possible, for once encysted,the parasites do no fu
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