SCOLECIDA. HI 



pelled die and decompose, and their contained eggs are thus 

 set free. Each egg (Fig. 39, 1) is covered with a little leathery 

 capsule which protects it from injury, and contains a minute 

 embryo in its interior. If this microscopically small egg be 

 swallowed as in many ways it easily may be by another 

 warm-blooded animal (in this particular case by the pig), 

 then a fresh series of changes ensues. The leathery case of 

 the ovum is dissolved in the stomach of the new host, and 

 the embryo is set free, when it bores its way through the 

 walls of the stomach by means of little siliceous hooks with 

 which it is provided. Having reached a suitable locality, the 

 young tape-worm proceeds to surround itself with a kind of 

 cyst, and it develops from its hinder end a kind of bladder 

 filled with fluid (Fig. 39, 2). It is now a bladder-worm, or 

 cystic worm, and as such would formerly have been regarded 

 as a distinct animal. In the particular case of the Tcenia 

 solium which we are now considering, the cystic worm is 

 found imbedded in the muscles of the pig, and it constitutes 

 in that animal the disease known as the measles. In this 

 cystic stage the young tape-woim rnay remain for an ap- 

 parently indefinite period, being quite incapable of develop- 

 ing eggs, though sometimes fresh bladder-worms may be 

 produced by a process of budding. For its further develop- 

 ment it is necessary that it should now be introduced into 

 the alimentary canal of man. If a portion of measly pork be 

 eaten with these cystic worms imbedded in it, then the 

 young tape-worm is liberated from its cyst: it fixes itself 

 by means of its suckers and booklets to the mucous mem- 

 brane of the intestine, and its caudal bladder drops off. It is 

 now converted into the head of the adult tape-worm. It 

 finally commences to throw out buds from its hinder extremity, 

 and in these buds or joints the reproductive elements are pro- 

 duced, so that ultimately we get the long, flattened jointed 

 colony with which we started. 



This extraordinary series of phenomena is now known to 

 occur in other cases, but space will not admit our dwelling 

 upon these. Another of the tape-worms of man (the Tcenia 

 mediocanellatd) is developed in the same way from the 

 measles of the ox. The tape-worm of the cat is the mature 

 form of the bladder- worm of mice, and the tape- worm of the fox 

 is derived from the cystic worm of hares and rabbits. Lastly, 

 man is not only liable to be infested with the tape-worms 

 derived from the cystic worms of other animals, but may be 

 attacked by the cystic or immature forms of the tape-worms 



