32 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



not an essential element in Radiolarian organization, is yet very generally 

 to be observed, namely the presence, in the strands of external protoplasm, 

 of numerous little rounded yellowish or greenish cells (Fig. 12, s.c) 

 apparently of extraneous origin which live and multiply sheltered within 

 the body of the Radiolarian. Here we have an excellent example of 

 what is known as symbiosis, in contradistinction to parasitism. In both 

 cases we have two different types of organism living in close association 

 e.g. the smaller living within or attached to the body of the larger. When 

 the benefit arising from this association is entirely one-sided the biologist 



Fig. 13. 

 Skeletons of RadiolariaDs. 



terms the beneficiary organism a parasite, when on the other hand the 

 benefit is mutual, when the smaller organism makes some return for the 

 hospitality it receives, the two organisms are said to be symbiotic. The 

 small cells living within the body of the Radiolarian receive from it 

 shelter and possibly a small amount of food-material, but on the other 

 hand they contain a colouring matter like that of green plants which 

 enables them to appropriate the carbon dioxide produced in the metabolism 

 of the Radiolarian, retaining the carbon for their own use but setting free 

 the oxygen to be made use of by the Radiolarian, so that we are clearly 

 justified in speaking of these small cells as symbiotic. 



Like the pelagic Foraminifera the Radiolaria play an important part 



