62 V Invertebrata. 



exposures to high temperature. On entering the 

 digestive organs of some animal with its food or diink, 

 the embryo is set free and travels through the tissues 

 of its new host as a little oval body armed in front 

 with weak hook-like or boring spines. On reaching 

 a suitable site it anchors, and the body dilates into a 

 sac full of water. In this cystic condition the animal 

 may remain stationary for a length of time, and by 

 budding the number of cysts is capable of a rapid 

 increase. When the flesh of an animal containing 

 such cysts is eaten by another, the liberated saccular 

 worm has its outer wall dissolved away, and its inner 

 portion lengthens and in a short time becomes a true 

 tapeworm. In most cases it requires two animals as 

 hosts for the proper perfection of the worm. Thus 

 the human tapeworm has its cystic stage in the flesh 

 of the pig, the condition of pork called ' measly ' being 

 due to these little cysts in the muscles of the pig. 

 Similarly, the tapeworm of the dog develops from 

 cysts found in the hare ; that of the cat from cystic 

 worms in the mouse, that of the fox from cysts in the 

 field-mouse, &c. 



In Ireland, the commonest human tapeworm has 

 four suckers but no hooks on its head (fig. 36 B), and is 

 known as Tcznia mediocanellata ; its larva inhabits 

 the ox. 



In Russia and Switzerland, the human tapeworm 

 is quite a distinct species, with very flat body, no 

 hooks, and two long grooves on its head in place of 

 suckers ; its larvae live in the waters of certain lakes, 

 and it has been supposed that it is through these 

 waters being used for drinking purposes that they gain 

 entrance into the human body. 



