Schneider: the phenomena of symbiosis. 943 



3. Individualism. — This form of symbiosis differs from 

 mutualism in that one or more of the symbionts is absolutely 

 dependent upon the other for its existence. It therefore rep- 

 resents a higher form of mutualism, from which it is no doubt 

 phylogenetically derived. Individualism may be divided into 

 semi-individualism and complete individualism. In the former 

 at least one of the symbionts is incapable of existing indepen- 

 dently; in the latter none of the symbionts can exist indepen- 

 dently. The associations form an individual, a morphological 

 unit, and the phenomena are frequently not recognized as sym- 

 biosis. Much of our knowledge in regard to individualism is 

 as yet purely hypothetical and theoretical. The subject there- 

 fore requires further careful study. 



( a. ) Semi- individualism. — This is perhaps of wide occurrence. 

 It is represented by the lower lichens in which the algal symbi- 

 ont is capable of leading an independent existence, while the 

 fungus can not. In the lowest crustaceous lichens there is per- 

 haps mere mutualism, since several investigators state that the 

 symbionts may live independently as fungus and alga. Another 

 instance occurs perhaps in those leguminous root-tubercles 

 formed by Rhizobium mutabile. At least there are no authentic 

 records to prove that this bacterium can grow and multiply in 

 artificial media. Some algae seem to form semi individualism with 

 animals. According to Kiihn, Pleurococcus brachypodis and Pleuro- 

 coccus chlopodis occur only upon the body (among the hair) of 

 the two and three-toed sloths. Simple-celled, chlorophyll- 

 bearing algae or chlorophyll-bodies have beec found in repre- 

 sentatives of the following genera of the animal kingdom: 

 Amoeba, Dactylospora, Difflugia, Hyalospliaenia, Heleopera, 

 Arcella, Cochliopodium. Actinosphaerium, Bhaphidiophrys, Ac- 

 anthocystis, Heierophrys, Chondropus, Sphaerastrum, Cilio- 

 plirys, Vorticella, Epistylis, Ophrydium, Vaginicola, Euplotes, 

 Urostyla, Uroleptus, Stichotricha, Spirostomum, Blepharisma, 

 Climacosiomum, Stentor, Cyrtostomum, Microfhorax, Paramec- 

 ium, Loxodes, Coleps, Lionotus, Amphileptus, Lacrymaria, 

 Phyalina, Holophrya, Euchelyodon, Enckelys, Spongilla, Hydra, 

 Vortex, Mesostomum, Hypostomum, Derosiomum, Couroluia, 

 Anthea, Bouellia, Idotea. In many instances the green par- 

 ticles occurring within the animals are simply remnants of 

 chlorophyll from the algae upon which the animal feeds. In 

 other instances there is an undoubted symbiotic association of 

 the alga and animal. 



