70 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



The ' 'facultative algae breeders" are interesting and signifi- 

 cant in our study of niicrosyinbiosis, in-as-much-as they, 

 apparently, furnished evidence of the influence of extrinsic 

 factors in the development of symbiosis. It is possible 

 that the particular species that fall in this group are repre- 

 sented by varieties in different localities, and that they do 

 not show any distinctive morphological features that dif- 

 ferentiate them from the type species. Such "varieties" 

 may have chemical and physiological variations which 

 could only be detected by experimental methods. It is 

 also possible that the microsymbiont may have different 

 properties in different localities. It appears more probable, 

 however, that the absence of the algal s>Tnbionts in certain 

 locahties is due to extrinsic factors, such as temperature, 

 oxygen, hght, pressure, etc. Certain cases of symbiosis, 

 which will be mentioned later, furnish evidence of the 

 influence of extrinsic factors. 



The hereditary transmission of the algal s^nnbionts is a 

 simple process in the protozoa, where reproduction consists 

 of simple fission of the parent organism. When the cyto- 

 plasm of the protozoan host divides, the algal symbionts 

 are distributed more or less equally between each of the 

 daughter animals. In the encystment stage of the proto- 

 zoan host, it has been observed by Penard ('90) and Leidy 

 (79) that in a number of the Foraminifera the algae are 

 retained within the body of the encysted animal. Gruber 

 ('04) also found algae within the cysts of Arnoeha viridis 

 that had been sent from America to Germany. In some 

 protozoa, however, the algal symbionts are expelled from 

 the body of the host before or during encystment {Acan- 

 thocystis aculeata), but the symbionts are retained in the 

 wall of the cyst. 



A number of investigators have observed that when the 

 host is in a state of starvation, the algal symbionts are 

 digested by the host animal. Greef ('75) observed a spon- 



