72 HOUSEHOLD INSECTS 



carefully and repeatedly traced through its life history, 

 and it has certainly been found to pass a part of its exist- 

 ence in man and a part in the body of the mosquito. 

 Moreover, the part of its life that is passed in man is not 

 like that passed in the mosquito, but both are necessary 

 to the ultimate existence of the germ. These facts have 

 been independently worked out by some of the world's 

 greatest scientists : Ross, Celli, Bignami, Daniels, Laveran, 

 Shipley, Bastianelli, and others. 



To those familiar with the lives and habits of the lower 

 animals it is not at all difficult to believe that one of them 

 can pass part of its life in the body of one animal and the 

 rest of its life in the body of a second animal. Many 

 cases of this kind are known and some of them commonly 

 known. For example, a common tape worm which exists 

 in the bowels of a human being spends a part of its life 

 in the body of a hog. In fact, we get this particular tape 

 worm only by eating what is known as measly pork. 

 That is, pork containing young minute forms of the tape 

 worm. The pork is eaten and the tiny, undeveloped tape 

 worm set free, which soon grows into an adult worm 

 within our own bodies. Again, there is the liver fluke 

 worm that causes the liver rot of sheep. This parasite 

 passes part of its life in a snail from which, after a time, 

 it crawls up on the blades of grass growing about ponds and 

 pools of water in which the snails live. In this situation 

 the minute worm is swallowed by the sheep along with 

 the grass and finally finds its way to the liver of the sheep. 

 Then, there is that much dreaded parasite, Trichinella 

 spiralis (Fig. 29). This is the little worm on which Uncle 

 Sam spends so much money hiring men to look for it in 

 the carcasses of animals in the great slaughtering and pack- 



