MICROSYMBIOSIS 69 



logical responses accompanying their invasions into animal 

 cells, algal sjTiibioses furnish important evidence of the 

 proposed mechanism by which Symbionticism operates in 

 the origin of new species. Furthermore, that type of 

 symbiosis in which algae are the definitive symbionts lends 

 itself to experimental analysis more readily than most cases 

 of bacterial micros^mbiosis. So far as Symbionticism is 

 concerned, there is no fundamental difference in bacterial 

 and algal symbiosis. Both types of symbionts, as well as 

 symbiosis in which other types of organisms are definitive 

 symbionts, will be considered in the following review. 



The cases of symbiosis in which bacteria are the definitive 

 symbionts, and which will be discussed in this chapter, 

 obviously, represent only those instances in which the bac- 

 terial nature of the symbionts have been readily recognized. 

 It has been suggested in a few instances, that the bodies 

 in question are mitochondria. Such confusion is rather to 

 be expected when we reaUze that mitochondria and bacteria 

 in general are one and the same thing, but showing wide 

 variations in morphology, physiologic response, staining 

 reactions, and responses to physical and chemical agents. 



Symbiosis in the protozoa, or one-celled animals, has 

 been studied mainly in the fresh-water forms, although 

 a few marine species are known to contain symbionts. 

 The majority of described cases of symbiosis in this phylum 

 deal with the more readily detectable algal symbionts, of 

 which there are numerous examples. Buchner divides the 

 protozoa into three groups from the standpoint of algal 

 symbiosis. The first group contains algal symbionts in 

 whatever locality they may be found. The second group 

 he calls "facultative algae breeders." This group is made 

 up of those species which contain algal symbionts in certain 

 localities, or under certain conditions. The third group 

 is made up of all remaining protozoa, and is characterized 

 as "algal enemies;" they never harbor algal sjTnbionts. 



