PARASITES OF ANIMALS. 83 



which this parasite grows, it is always liable to produce se- 

 rious disease, or even death; but the symptoms that it 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 by 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 (Tcenia marginata) resembles the pork 

 tape-worm of man (Tcenia 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 various parts of the body. Many lodge in the 



