THE WONDERS OP LIFE 



generation by parasitic habits in a number of different 

 orders and families. 



From parasitism we must entirely distinguish that 

 intimate life-union of two different organisms which we 

 called symbiosis or mutualism. Here we have an as- 

 sociation of two living things for their mutual benefit, 

 while the parasite liv^ entirely at the expense of his 

 host. Symbiosis is found among the protists, being 

 very wide-spread among the radiolaria. In the gelati- 

 nous envelope (calymma) which encloses the central cap- 

 sule of their unicellular bodies we find a number of 

 motionless yellow cells {zooxanthella) scattered. These 

 are protophyta or (as it is said) "unicellular algas" of 

 the class of paulotomea {palmellacea). They receive 

 protection and a home from the radiolaria, grow plas- 

 modomously, and multiply by rapid segmentation. A 

 large part of the starch-flour and the plasm which they 

 form by carbon - assimilation goes as food directly to 

 the radiolarium-host; the other part of the xanthella 

 goes on growing and multiplying. Similar yellow zoo- 

 xanthella or green zoochlorella are found as symbionta 

 in the tissues of many animals. Our common fresh- 

 water polyp (hydra viridis) owes its green color to the 

 zoochlorella which live in great numbers on the ciliated 

 cells of its entoderm (the digestive gut-epithelium). In 

 general, however symbiosis is rarer in the metazoa 

 than in the metaphyta. In the latter case it is the 

 fundamental feature of a whole class of plants, the 

 lichens. Each lichen consists of a plasmodomous plant 

 (sometimes a protophyte, sometimes an alga) and a 

 plasmophagous fungus. The latter affords a home, 

 protection, and water to the green alga, which repays 

 the service by providing food. 



