2/0 BACTERIOLOGY. 



requires a complete and more or less extensive 

 alternation of generations. Such a parasite, in 

 other words, attacks different animals and 

 plants, one after another, in each of which it 

 passes through a definite stage of development. 

 Among most parasites there occurs a free or 

 saprophytic stage which is advantageous to 

 the maintenance of the species. In such a 

 case the parasite is often autoecious, that is, no 

 change of host occurs. In alternation of gen- 

 erations there is always change of host or 

 hetercecism, so that in the extreme cases of 

 strongly obligatory parasitism this occurs in 

 a fashion as if saprophytism were altogether 

 omitted. In the tape-worm, for example, we 

 know no free stage ; the eggs pass into the 

 outer world but do not develop (at least up to 

 the present nothing of the sort is known). The 

 scolex of Tcznia solium is found in hogs, the 

 tape-worm in man ; the scolex of Tcznia 

 mediocanellata is found in cattle, the tape- 

 worm belonging to it in man ; the Echinococci 

 are found in man, the respective tape- worm in 

 dogs. The scolex of Bothriocephalus latus is 

 found in predaceous fishes, and the tape-worm 

 in man, but free living embryos arise from the 

 eggs in water. In this latter case, therefore, 

 a limited free stage does occur. In Distomum 



