72 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



only in one genus, Pelomyxa. The bacteria are rod-shaped 

 and vary in size in the different species. In Pelomyxa 

 vim-para they are grouped close to the nucleus, reminding 

 one of the oft-observed arrangement of mitochondria. In 

 other species they are scattered throughout the cytoplasm 

 just as mitochondria often are. These symbiotic bacteria 

 measure 10-15 micra in length in some species, while in 

 other species they may be from 40-50 micra long. The 

 physiologic significance of these symbionts has not been 

 determined. 



In the Sponges, algal symbiosis is fairly common, particu- 

 larly in the fresh-water species. More than one species of 

 algae may be present as symbionts. Some sponges con- 

 tain only single-celled algae within their cells; others harbor 

 the more complex multicellular forms. In some species 

 of sponges, the symbionts are represented by a varying 

 algal flora. The physiologic and morphologic responses in 

 these symbiotic relationships do not appear to have been 

 investigated. No reference to bacterial symbionts in the 

 sponges has been found. 



Symbiosis in the Coelenterates has been widely known 

 and studied through familiarity with the fresh water form 

 Hydra viridis. Buchner gives a table of the species in which 

 symbiosis has been recognized. The hst includes a large 

 nimiber of species of Hydrozoa, Scyphozoa, Anthozoa and 

 Ctenophora. The definitive symbionts in all these symbiotic 

 relationships are zoochlorellae and zooxanthellae (green 

 and yellow algae). In the more simple Hydrozoa the 

 symbiotic algae are located chiefly in the entoderm. 

 There is a variation, however, in their distribution within 

 this layer. The symbionts are usually absent in the 

 lower part of the entodermal cavity where the cells are 

 vacuolated. They are present, in most cases, in the ento- 

 derm of the tentacles. In the higher coelenterates the 

 symbionts are more widely distributed. They are also 



