Fig. 139.—Egg of the marginate tapeworm (Tænia marginata), with six-hooked embryo, greatly magnified. (Stiles, Annual Report, U.S.A. Bureau of Agriculture, 1901.)
Fig. 140.—Portion of the liver of a lamb which died nine days after feeding with eggs of the marginate tapeworm (Tænia marginata), with numerous “scars,” due to young parasites. (After Curtice.)
Life history. In tracing the life history it is best to begin with the egg, produced by the adult tapeworm in the intestine of dogs. These eggs, containing a six-hooked embryo, escape from the dog with the excrements, and are scattered on the ground, either singly or confined in the escaping segments of the tapeworm. Once upon the ground, they are easily washed along by rain into the drinking water, ponds, or brooks, or scattered on the grass. Upon being swallowed with fodder or water, they arrive in the stomach of the intermediate host (cattle, sheep, etc.), where the eggshells are destroyed and the embryos set free. The embryos then traverse the intestinal wall, and, according to most authors, arrive either actively, by crawling, or passively, by being carried along by the blood, in the liver or lungs, where they undergo certain transformations in structure. While still in the finer branches of the blood-vessels of the liver, which they transform into small, irregularly shaped tubes about 12 to 15 mm. long and 1 to 1·5 mm. broad, the embryos lose their six hooks, and develop into small, round kernels, which are generally situated at one end of the tubes. The embryo can first be seen about four days after infection. The “scars” (Figs. 140 and 141) described in the liver of animals infested with Cysticercus tenuicollis are nothing more nor less than these tubes, or altered blood-vessels, caused by the growth and wandering of the parasites.
Curtice takes a somewhat different view—that is, he considers the liver as a place of destruction for the young parasites, rather than a normal place for their development; he also claims that the embryos, which may even travel the entire length of the intestine of the intermediate host, traverse the intestine and arrive directly in the position where they complete their larval development without first passing through the liver.
Fig. 141.—Cross-section of the liver of a lamb which died nine days after feeding with eggs of the marginate tapeworm (Tænia marginata). (After Curtis.)
After developing into the full-grown bladder-worm, the parasites remain unchanged until they are devoured by a dog or wolf, or until, after an undetermined length of time, they become disintegrated and more or less calcified.
If the hydatid is devoured by a dog or wolf, either when the latter prey upon the secondary host or when the dog obtains the cyst at a slaughter-house, the bladder portion is destroyed, the scolex alone remaining intact in the digestive fluids. The head holds fast to the intestinal wall with its suckers and hooks; by strobilation (transverse division) it gives rise to the segments, which as we have already seen, together with the head, go to make up the adult tapeworm. Reproductive organs of both sexes develop in the separate segments, and eggs are produced, within which are developed the six-hooked embryos, the point from which we started.