I20 INVERTEBRATE ANIMALS. 
of the colony break off, and are expelled from the alimentary 
canal of the host. The joints thus expelled die and decompose, 
and their contained eggs are thus set free. Each egg (fig. 50, 
I) 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 par- 
ticular case by the fzg), 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 tapeworm proceeds to surround itself with a 
kind of cyst, and it develops from its hinder end a kind of blad- 
der filled with fluid (fig. 50, 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 Tenia solium 
which we are now considering, the cystic worm is found imbed- 
ded in the muscles of the pig, and it constitutes in that animal 
the disease known as the measles. In this cystic stage the. 
young tapeworm may remain for an apparently indefinite period, 
being quite incapable of developing eggs, though sometimes 
fresh bladder-worms may be produced by a process of budding. 
For its further development 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 tapeworm is liberated from its cyst : it fixes it- 
self by means of its suckers and hooklets to the mucous mem- 
brane of the intestine, and its caudal bladder drops off. It is 
now converted into the head of the adult tapeworm. It finally 
commences to throw out buds from its hinder extremity, and in 
these buds or joints the reproductive elements are produced, 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 tapeworms of man (the Zenda 
medtocanellata) is developed in the same way from the measles 
of the ox. The tapewerm of the cat is the mature form of the 
bladder-worm of mice, and the tapeworm of the fox is derived 
from the cystic worm of hares and rabbits. Lastly, man is no 
