Chapter 33 



PARASITISM AND PATHOGENESIS; SYMBIOSIS; 

 COMMENSALISM; GREGARIOUSNESS AND 

 COMMUNAL LIFE; PREDACIOUSNESS; 

 INSECTIVOROUS PLANTS; EPIPHYTISM; 

 SAPROPHYTISM 



There are many kinds of biotic relationships in the living world, ex- 

 tending from the more or less dependence of living organisms on each 

 other to the more or less independence, or even antagonism, in other 

 organisms. These relationships may exist between different species of 

 animals, different species of plants, or between animals and plants. The 

 following brief descriptions of some of these relationships are representa- 

 tive: 



I. PARASITISM AND PATHOGENESIS 



In parasitism (par' a sit izm) (Gr. para, beside; sitos, food) an organ- 

 ism known as the parasite lives on, or within, and at the expense of, an- 

 other living organism known as the host. In this condition the host may 

 not be killed immediately (contrast with predaciousness and insectivorous 

 plants). When the parasite lives on the outside of the body of the host 

 it is an ectoparasite (Gr. ektos, outside) ; when it lives within the body of 

 the host, it is an endoparasite (Gr. endo, within). In parasitism the host 

 is harmed while the parasite benefits. 



When the effects of parasitism on the host result in discernible, ab- 

 normal characteristics (symptoms), we may consider the condition as 

 disease-production, or pathogenesis (path o -jen' e sis) (Gr. pathos, dis- 

 ease or suffering; genesis, origin). In some cases the distinction between 

 parasitism and pathogenesis may be slight, but when actual, discernible 

 disease-production results, we may consider it as pathogenesis and the 

 parasite which causes the disease as a pathogen. 



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