THE MUTUAI^ RELATIONS OF ORGANISMS 231 



attach themselves to the host permanently. A third type, like 

 the lice, lives wholly upon its host and has no life apart from it. 

 While these ectoparasites may be trouble- 

 some, they are not especially injurious, 

 except when they transmit disease germs. 

 Endoparasites. Parasites that live 

 within the body of the host are endo- 

 parasites. They are numerous and pro- 

 duce far more mischief than ectoparasites. 

 Among them are those that produce vari- 

 ous deadly diseases like trichinosis (Fig. 

 108), tuberculosis, diphtheria, etc. 



The Effect of Parasitism 



Parasitism occurs among both animals 

 and plants. The number of species of 

 parasites is very great, but cannot be 

 estimated. Nearly all species of animals 

 and plants have their own parasites, and 

 some have several species of parasites 

 infesting them. For this reason it is 

 sometimes stated that there are at least 

 as many species of parasites as there 

 are species of non-parasitic organisms. 

 The effect of the parasitism upon both 

 host and parasite is profound, but natu- 

 rally quite different. 



Upon the Host. The parasite usually 

 injures the host and is then spoken of as 

 pathogenic (Gr. pathos = disease + -geneia 

 = producing). The amount of injury varies widely. In some 

 cases, the parasite produces disease and even the death of 

 the host. Trichina is a parasitic worm (Fig. 108), which occa- 

 sionally causes trichinosis in man, resulting sometimes in death. 

 Certain flies occasionally make their way into the skull cavities 



"A B 



FIG. 108. TRICHINA 



A, a single worm showing 

 its internal anatomy; B, 

 worm coiled up in a bit of 

 muscle of pork. If uncooked 

 pork containing these worms 

 is eaten they are set free in 

 the intestines and a case of 

 trichinosis results. 



