INFECTION VIA FOOD 5 



have been expelled from the body of a former host. 

 The method of infection is said to be a casual or con- 

 taminative one, and there is always the possibility of 

 the resting parasite never resuming activity owing 

 to it never being absorbed with food by the host, 

 together, of course, with the equal possibility of its 

 rapid transfer to another higher animal. 



Three variations of the casual method of infection 

 may be encountered. In the first case the parasite 

 does not inflict serious damage directly upon the 

 host, but injures the larger animal by depriving it of 

 a small amount of nourishment. Certain parasitic 

 Protozoa, known as Gregarines, live in the ali- 

 mentary tract of some marine worms without 

 damaging the food canal itself. They are free in the 

 cavity or lumen of the intestine, migrate throughout 

 its length, and absorb nutritive substances from the 

 mass of food with which they are in contact. 



Other members of this same group of Protozoa, 

 the Gregarines, not content with simply absorbing raw 

 food, seek to obtain some food already manufactured. 

 To secure this, one end of the body of the Protozoon 

 becomes applied to the wall of the gut, and may 

 even bore in between the elements composing it. 

 Consequent on this action, some of the living proto- 

 plasm is destroyed, and the almost liquid contents of 

 the injured cells pass into the parasite. The portion 

 of the Protozoon which is between the cells serves 

 thus the double purpose of securing the parasite 

 as it feeds, and of aiding in the feeding process. 



Yet a further stage of dependence is found in the 

 case of the parasites (Coccidia) causing the disease 



