SYMBIOLOGY THE BIOLOGICAL RELATIONSHIPS OF ORGANISMS 149 



genetic generations. Many are limited to one host-species, or even to 

 definite tissues or organs. 



One organism may enter into different forms of symbiosis. For 

 example, the bacillus of typhoid fever may enter into an accidental 

 (perhaps contingent) symbiosis with the oyster, while with man it forms 

 an antagonistic symbiosis. The bacillus of Asiatic cholera, likewise, may 

 live in and upon various animals without any injurious effects, but as soon 

 as it finds its way to the intestinal canal of man it acts as a true parasite. 

 The differentiation into facultative and obligative parasitism depends 

 upon the ability that some organisms have of living as parasites and sapro- 

 phytes, while others are absolutely dependent for their existence upon 

 association with the living host. 



The most common parasites are the fungi. The Schizomycetes form 

 antagonistic symbioses, preferably with animals. The higher parasitic 

 fungi predominate upon vegetable tissue. Many diseases of animals 

 are also due to the higher fungi. Algae occur parasitically in and upon 

 plants and animals. Many of the Chlorophyceae 'and Cyanophyceae 

 occur as parasites upon higher plants. Many of the marine algae are 

 parasitic upon each other as well as upon marine animals. Higher plants 

 are often parasitic (Mistletoe, Dodder, Indian Pipe, etc.). Protozoa occur 

 parasitically upon animals. A sporozoan causes malaria. Still higher 

 animals occur parasitically in and upon animals and plants, producing 

 manifold injurious effects. 



Most interesting is the phenomenon of sex-parasitism in which one 

 sex, usually the male, lives parasitically upon the other. In one of the 

 parasitic crustaceans the male is entirely dependent upon the female for 

 its sustenance. Among the Bouellias the male is represented by a mere 

 fertilizing structure, parasitic within the reproductive organs of the female. 



We may also mention the parasitic relationship of embryos and the 

 mother-organisms. This has already been referred to as a questionable 

 form of symbiosis. Klebs is, however, of the opinion that it is true para- 

 sitism. The embryo of a plant derives all its nourishment from its parent, 

 and in addition takes from it certain materials which it stores for future 

 use (cotyledons, endosperm). Even after birth the young of many ani- 

 mals remain in parasitic association with the parent. Of the numerous 

 eggs of the black salamander, only one develops a young animal, which 

 eats the remaining eggs. 



3. Saprophytism. Saprophytism is not true symbiosis. This is a 

 condition which in many instances was no doubt phylogenetically de- 

 rived from parasitism as we have all gradations between obligative para- 

 sites and obligative saprophytes. It is quite reasonable to assume that in 

 many cases of parasitism in which the death of the host was the final 



