154 FORAMINIFERA, RADIOLARIA, DIATOMS less. 



protoplasm are found certain little rounded bodies of a 

 yellow colour, often known as " yellow cells " (Fig. 33, z). 

 Each consists of protoplasm surrounded by a cell-wall of 

 cellulose, and coloured by chlorophyll, with which is asso- 

 ciated a yellow pigment of similar character called diatomin. 



For a long time these bodies were a complete puzzle to 

 biologists, but it has now been conclusively proved that they 

 are independent organisms resembling the resting condition 

 of Haematococcus, and called Zooxanthella nuiricola. 



Thus an ordinary Radiolarian, such as Lithocircus (Fig. 

 33), consists of two quite distinct things, the Lithocircus in 

 the strict sense of the word plus large numbers of Zooxan- 

 thellae associated with it. The two organisms multiply quite 

 independently of one another : indeed Zooxanthella has 

 been observed to multiply by fission after the death of the 

 associated Radiolarian. 



This living together of two organisms is known as sym- 

 biosis. It differs essentially from parasitism (see p. 123), in 

 which one organism preys upon another, the host deriving 

 no benefit but only harm from the presence of the parasite. 

 In symbiosis, on the contrary, th6 two organisms are in a 

 condition of mutually beneficial partnership. The carbon 

 dioxide and nitrogenous waste given off by the Radiolarian 

 serve as a constant food-supply to the Zooxanthella : at the 

 same time the latter by decomposing the carbon dioxide 

 provides the Radiolarian with' a constant supply of oxygen, 

 and at the same time with two important food-stuffs — starch 

 and proteids — which, after solution, diffuse from the protoplasm 

 of the Zooxanthella into that of the Radiolarian. The 

 Radiolarian may therefore be said to keep the Zooxanthellge 

 constantly manured, while the Zooxanthellae in return sujDply 

 the Radiolarian with abundance of oxygen and of ready- 

 digested food. It is as if a Haematococcus ingested by an 



