The science of life : fully illustrated in tone and line and including many diagrams

FURTHER PATTERNS OF INDIVIDUALIZED ANIMAL LIFE

worms become anesthetized by the chemical products of their own activity and sink into a drugged immobility. But when the host consumes a meal the blood-supply to his gut is increased enormously and oxygen diffuses—accidentally so far as he is concerned—into the intestinal canal. When this happens the roundworms revive and begin to gulp; when his digestion is over they sink again into their torpid sleep. There is a rhythm in the life of an intestinal roundworm as marked as our own rhythm of day and night ; a period when there is nothing doing, and then a period of activity, when the world in which it lives begins to churn and writhe, and when oxygen and food are suddenly abundant. Instead of dawning and waning light there is dawning and waning oxygen. It is an easy life ; there is nothing to fear (except the death of the host) and nothing to do but sleep and swallow ; the weather is always perfect. Fortunately, having no brain worth mentioning, the worm cannot possibly get bored. But there is another side to the life of a parasite. Before it can enjoy this Elysium it has to get there, and it is no easy thing for a worm as simply made as the roundworm to invade a mammalian intestine. Consequently we find that eggs are produced in enormous numbers—a single Ascaris, it has been reckoned, can produce 15,000 eggs a day —and only very few among these thousands reach their earthly paradise. In some species, such as the comparatively harmless Ascaris lumbricoides of the human bowel, the eggs pass out with the feces, and a few lucky ones are swallowed by a new host—floating, for example, in his drinking water—and thus win through. It is simply a matter of hit and miss, and the prolific mother makes so many shots that they cannot all miss. In other species, however, there are more elaborate histories. In the dreaded Trichinella spiralis, for example—a worm only a few millimetres long—the adult lives and multiplies in the intestine of a man, a pig, a rat, or another suitable mammal. The fertilized female burrows into one of the villi and there brings forth living young—sometimes many thousands—which escape into the lymphatic vessels of the host. Thus, reaching the circulation, they are carried all over the body, until they settle down in the voluntary muscles. They burrow into the muscles, shut themselves up in little hard cases and wait. Nothing more happens to them until the host dies ; but if after his death his flesh is eaten by another mammal—a man by a

rat, or a pig by a man—they revive and flourish in the bowels of the latter. An infected animal may have tens of millions of these parasites patiently waiting in his muscles for his death, and hurrying it up by producing the inflammations or other symptoms of “trichiniasis.’” It has been calculated that one ounce of infected or “ measly”? pork contains 85,000 of them—if the pork was not properly cooked its consumption would involve the liberation of all these parasites in the bowel of the eater, and if half of these were females, producing on the average a thousand embryos each, the result would be a migration of forty million young worms into his muscles.

Again in the guinea-worm, Filaria medinensis—a worm from one to six feet long—the adults live in the connective tissue of man, just under the skin. The young escape into the outer world through abscesses caused by the presence of the parasite, and some of them have the good fortune to get into a stream ora

pond

Here they

make their

way into Te

the bodies Fig. 91. Microscopic view of a of water- piece of “measly”? pork, showing fleas, Cy- white, parasitic worms (Trichiclops, where nella) curled up among the musclethey wait fibres.

until the

water is drunk by a man careless enough not to filter it first. Thus they reach the human bowel, and thence, presumably by the circulation, they travel to the connective tissue of the skin. In a case of this kind, where the adults live inside a creature of one species, but the young, 1n order to get there regularly, enter the bodies of some quite different animal, the latter 4s called an intermediate host.

We cannot enter here into an account of the many peculiar features of the organization of the roundworms which has led to their being put apart in a phylum of their own, In a number of respects they are interesting —their spermatozoa, for example, instead of swimming by means of long, lashing tails, creep slowly like white blood corpuscles ; and their epidermis, instead of being divided

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