MICROSYMBIOSIS 71 



taneous expulsion of the symbionts in Acanthocystis, in the 

 vegetative stage and a subsequent re-entrance of the sym- 

 bionts into the animal body. 



Seasonal variations in the occurrence of algae in a host 

 have been described by Balbiani, Roux, and Wesenberg- 

 Lund. Roux observed that Stentor polymorphous har- 

 bored algae in the spring of the year, while in August, 

 September and October, the animals did not contain any 

 algae. The experunents of LeDantec ('92) are significant 

 in connection with a study of the physiological responses 

 under the influence of extrinsic factors. He placed Par- 

 amecium hursaria in the dark, and found that after twenty- 

 four hours the algae had become brown in color. When 

 kept there for a longer period, the algae were found to be 

 in a state of digestion. "When the animal is later returned 

 to the light and kept in sterile water, the algae do not re- 

 appear; it is necessary for a reinfection to take place. 



An interesting example of the apparent influence of 

 extrinsic factors on symbiosis is to be found in the well 

 known luminiferous Noctiluca milearis. This animal is 

 quite widely distributed, and usually does not contain any 

 algal symbionts. AI. and A. Weber ('90-'91) found this 

 animal in the Bay of Bima on the Island of Sumbawa 

 (Indian Ocean) so numerous and so filled with green algae 

 that the surface of the water appeared green. They could 

 find no evidence of digestion of the symbionts. 



Zooxanthellae, or "yellow algae," have been described in 

 a large number of protozoa. These forms have been ob- 

 served to produce "spores" while in the symbiotic state. 

 These spores are apparently retained in the cytoplasm of 

 the host as small yellow or brown granules. Zooxanthellae 

 in the free-Uving state are often provided with two flagella 

 at one end of the body. These flagella are left behind when 

 the yellow algae enter the body of a host. 



Bacterial symbionts in the protozoa have been described 



