Parasitism and Disease 



13 



sometimes to the injury caused by the work of 

 parasites in the appendix. 



Parasites may cause morphological or struc- 

 tural changes in the tissues of their hosts. The 

 stimulation caused by their presence may result 

 in swellings or excresences or other abnormal 

 growths. Interesting examples of this are to be 

 found in the way in which pearls are formed in 

 various mollusks. In the pearl oysters of Ceylon 

 occur some of the best pearls. If these are care- 

 fully sectioned there may usually be found at the 

 center the remains of certain cestode larvae whose 

 presence in the oyster caused it to deposit the 

 nacreous layers that make up the pearl. Other 

 parasites cause similar growths in various shell- 

 fish. The great enlargements of the arms or legs 

 or other parts of the body seen in patients affected 

 with elephantiasis is an abnormal growth due to 

 the presence of the parasitic filarae in some of the 

 lymph-glands where they have come to rest. 



Finally, the parasite may exert a direct physio- 

 logical effect on the host. This is evident when the 

 parasite demands and takes a portion of the 

 nourishment that would otherwise go to the build- 

 ing up of the host. Sometimes this is of little im- 

 portance, but at other times it may be a matter of 

 life or death to the infected animal. The physio- 



