296 



THE WONDER OF LIFE 



by the Radiolarian, and are able to build up carbon- 

 compounds, such as starch. They give off oxygen, 

 which is of course profitable for the animal, and they 

 doubtless utilize nitrogenous waste products made by 



the animal. If 

 things are not going 

 well, it is always 

 open to the Radio- 

 larian to digest its 

 partners ! The huge 

 numbers of Radio- 

 larians alike of in- 

 dividuals and of 

 specie s seem to 

 indicate that the 

 symbiosis is very 

 profitable. 



The symbiotic 

 Algae are known as 

 Zooxanthellae, and 

 their occurrence has 



FIG. 50. The Green Hydra, expanded and been recorded in a 

 contracted. The expanded form shows 

 the male organs or testes as swellings 

 near the base of the tentacles and the 

 ovary as a swelling near the attached 

 base. The transparent ectoderm is 

 seen as a layer surrounding the endoderm they are very abun- 

 which contains the green elements, " . 



usually regarded as symbiotic Algse. dant and quite in- 



dispensable. Prof. 



Keeble has shown that the larvae do not develop unless 

 they are infected, and that an adult which has been 

 driven by straitened circumstances to absorb its partners 

 can be re-infected and given a new lease of life. 



variety of an i ma ls. 



J 

 In the Planarian 



n , 



worm tonvoluta, 



