Tapeworms

Many modifications occur in the form of the bladder-worm, but the essential features of the development are the same throughout the order. In some forms the original bladder-worm gives rise to several or many scolices instead of the usual single scolex, a phase of asexual multiplication thus being introduced.

In the gid-parasite of the sheep (Taenia or Multiceps inu/ticeps) the bladder-worm, which occurs in the brain and is often called a Coenurus, gives rise to numerous scolices, each capable of develop ing into an adult worm if ingested by a dog or other suitable car nivorous host.

In Echinococcus granulosus this phase of multiplication is elab orated still further, the original embryo developing in the inter mediate host into a large and relatively thin-walled bladder or cyst ("hydatid"), from the inner surface of which there are budded off a large number of "brood-capsules," each containing from z o to 3o scolices. Daughter cysts may also be budded off, and these may give rise to further scolices, so that altogether a very large num ber of potential adult worms may be formed from the original em bryo. This worm occurs, in the hydatid stage, in the liver and other organs of a large number of hosts, including man, sheep, ox and pig. The adult is a very small worm with only three or four segments, and occurs in the intestine of the dog, wolf and fox.

As may be noted from the examples already given, the inter mediate host is usually an animal likely to be devoured by the final host. Thus the dog harbours several Taenioid tapeworms whose intermediate hosts are rabbits and sheep. The cysticercus of Taenia taeniaeformis of the cat occurs in rats and mice. A number of species of Hymenolepis parasitic, as adults, in ducks and geese make use of small fresh-water Entomostraca such as Cyclops and Cypris as intermediate hosts. Houseflies and earthworms are the vehicles of infection for several of the tapeworms of fowls. Dipy lidium caninum, a common parasite of the dog and cat, passes its cysticercus stage in fleas and in the louse, Trichodectes. The louse is capable of swallowing the eggs of the worm, but the adult flea cannot do so. Fleas become infected during their larval period, the eggs, when swallowed, hatching in the hinder part of the in testine and the embryo boring through its wall into the body cavity. Here they do not continue their development until the

pupal stage has been passed, the cysticerci becoming fully formed only in the adult flea. Infection is acquired by dogs and cats, and accidentally by man, through swallowing fleas containing them.

The larvae of many of the Phyllobothriinea, whose adult forms are found in the spiral valve region of the intestine of sharks and rays, occur among the tissues of other marine animals. One such larval form (apparently belonging to a species of Tylocephalum) occurs in the pearl-oyster, and is believed to be one of the prin cipal causes of the production of pearls. This, apparently, takes place only in the event of the death of the parasite from some un known cause.

Economic Importance.

From the fact that tapeworms often occur in enormous numbers in the intestine of an apparently perfectly healthy animal, it is clear that they are not always definitely injurious to their hosts. As parasites of man and of domestic animals, however, they have considerable importance. As a rule their ill effects are most manifest in young animals, and they are sometimes responsible for serious digestive and nervous disturbances, and even for severe losses among stock and poultry. The adult worms, besides depriving the host of a certain amount of nutrient material, secrete substances which have been shown to be definitely toxic in many instances, when injected into the blood or body-cavity of experimental animals. The same is true of the larval forms, and certain changes take place in the blood-cells of infested animals which indicate that some of these poisons are normally absorbed into the system. It is probable, however, by the migrations of the larvae within the body, and the secondary invasions by pathogenic organisms for which they prepare the way, that the most serious harm is done. Only in exceptional cases, such as the invasion of the brain by the hydatid of Echinococcus, are tapeworms really dangerous to man. (H. A. B.)