142 ELEMENTARY ZOOLOGY 



death, to the host. Such infested muscle or flesh is said 

 to be "trichinosed," and the flesh of a trichinosed 

 human subject has been estimated to contain 100,000,000 

 encysted worms. To complete the development of the 

 encysted and sexless Trie hince the infested flesh of the 

 host must be eaten by another animal in which the worm 

 can live, e.g. the flesh of man by a pig or rat, and that 

 of a pig by man. In .such a case the cysts are dissolved 

 by the digestive juices, the worms escape, develop repro- 

 ductive organs and produce young, which then migrate 

 into the muscles and induce trichinosis as before. But 

 however badly trichinosed a piece of pork may be, 

 thorough cooking of it will kill the encysted Trichina, so 

 that it may then be eaten with impunity. Some people, 

 however, are accustomed to eat ham, which is simply 

 smoked pork, without cooking it, and in such cases there 

 is always great danger of trichinosis. 



Wheel animalcules (Rotifera). TECHNICAL NOTE.- Live 



specimens of Rotifers can be found in almost any stagnant water 

 Examine a drop of such water with the compound microscope, and 

 find in it a few small, active, transparent creatures, larger than the 

 Paramcccium and other Protozoa in the water and which have the 

 appearance shown in fig. 34. They may be known by the constant 

 whirling, or rather vibrating, circlet or wheel of cilia at the larger 

 or head end of the body. These wheel animalcules may be studied 

 alive by the class. Although usually darting about, the animalcules 

 occasionally cease to move, when, because of their transparency, 

 almost the whole of their anatomy can be made out. Their feeding 

 habits can also be readily observed, and the food itself watched 

 as it moves through the body. Make drawings showing as much 

 of the anatomy as can be worked out. Note especially the "mas- 

 tax " or gizzard-like masticating apparatus in the alimentary canal. 



The wheel animalcules (fig. 34) or Rotifers look little 

 like the other worms we have studied. But they are 

 nevertheless more nearly related to the worms than to 

 any other branch of animals. They are all small, about 

 mm. long, and have a compact body. They are aquatic 

 and feed on smaller animals and plants or on bits of or- 



