62 Invertcbrata. 



exposures to high temperature. Being set free from 

 the capsule on entering the digestive organs of some 

 animal with its food or drink, the embryo, a little oval 

 body armed in front with weak hook-like or boring 

 spines, travels through the tissues of its host. On reach- 

 ing 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 in- 

 crease. When the flesh of an animal containing such 

 cysts is eaten by another, the liberated saccular worm 

 loses its outer wall, and its inner portion lengthens and 

 in a short time becomes a true tapeworm. In most 

 cases the life cycle requires two kinds of animals as 

 hosts, one in which the larva or scolex state is pro- 

 duced and one for the perfected 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 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 Tania mediocanellata ; its larva inhabits 

 the muscles of 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 their ciliated embryos develop within the bodies 

 of fresh-water fishes, and thence they gain entrance 

 into the human body, 



