PARASITES OF ANIMALS. 83 
which this parasite grows, it is always liable to produce se- 
rious disease, or even death; but the symptoms thatit causes 
and the danger will depend very much upon the situation of the 
tumors. In some districts it is extremely common. According 
to Dr. Thudichum, it occurs in nearly every sheep slaughtered 
in London ; and the dogs that feed upon the offal of slaughter- 
houses nearly always have the mature tape-worms in abun- 
dance. 
When a dog swallows one of these bladders, either free or 
in its cyst, the bladder portion is digested, and the head be- 
comes protruded ; on reaching the intestine, it fastens itself to 
the membrane hy means of its suckers and hooks. In this 
situation, it rapidly develops new joints, and in the course of 
three or four months becomes a mature tape-worm, about 
three feet in length, and begins to discharge its ripe joints 
filled with eggs. A dozen or more of these tape-worms may 
exist together in the intestine of one dog. In general appear- 
ance, this tape-worm (Zenia marginata) resembles the pork 
tape-worm of man (Ztenia solium), but never grows so large, 
and its neck portion is much thicker, compared with the size 
of the head as shown in Figure 61. 
The first hundred joints are very short ; the mature joints are 
squarish, the posterior end of one somewhat overlapping the 
anterior end of the next. These joints contain a very much 
branched and subdivided female organ, with an oviduct ter- 
minating on one edge in a bell-shaped orifice ; and an arbor- 
escently branched male organ or spermary, with small, round 
dilations connected with the small branches. Dogs harbor- 
ing these tape-worms scatter the mature joints and thousands 
of eggs everywhere over the fields, and in the water of 
streams and ponds. ‘The sheep and cattle swallow the eggs, 
either with their food or water or both. The eggs are hatched 
in the stomach of the sheep, and liberate minute worms, 
which are armed with three pairs of hooks for boring their 
way through the tissues, like the embryos of other tape-worms. 
By this means, these embryos force their way through the 
lining membrane of the intestine into the blood-vessels, and 
are carried to varioumparts ofithebody. Many lodge in the 
