WORMS. 
117 
Fig. 8. 
events which we find in Tmnia solium, a common Tapeworm of man 
in Northern Europe. This worm is matured in the intestines of 
man ; its final joints consist merely of 
fertilized ova which have already passed 
through the earlier stages of development ; 
when the joints are detached and dis- 
charged, their contents escape in the form 
of embryos contained in a thick chitinous 
shell. If these are now swallowed by a 
pig, the shell is digested by the gastric 
juices of the new host, and a rounded 
embryo, which is provided with three 
pairs of hooks, is set free ; by means of 
these hooks the guest makes its way 
through the wall of the stomach or 
intestine, and finally settles down in the 
muscles of its host. The embryo now 
loses its hooks, and gradually acquires a 
bladder-like form, the central cavity of 
which is filled with fluid. This bladder- 
worm (Cysticercus) has its outer wall 
pushed inwards at the anterior end, and 
on this hooks and suckers become de- 
veloped. We have now a narrow head 
and neck with an attached bladder, the 
head being at this time hollow. If during 
the long time that these bladder- worms 
remain alive, the pig is killed for food, 
its flesh is found to be " measly " ; if it 
is afterwards insufficiently cooked and 
eaten, the worms are conveyed into the 
human stomach. Here the bladder-like 
termination becomes absorbed, and, the 
neck beginning to grow, we have the commencement of the form 
from which we started, and the completion of that " vicious circle " 
which is so curious a characteristic of many forms of parasitic life. 
In other Tapeworms the cyst may be more complicated than that 
in the pig, as, for example, the form found in the sheep's brain or 
the liver of the horse. 
Of the other Cestode parasites mention should specially be made 
of those of Fishes ; the vulgar notion that the parasites of these 
Txnia solium : showing the 
head Qi) with its suckers 
(s') and crown of hooks 
(s), the unjointed neck 
(w), and a few of the 
succeeding joints (j). 
