TV] MODES OF NUTRITION 101 



neither organism being able to exist without the 

 other. This, however, is not always the rule, and it 

 is probable that the association may be one which 

 can be lost and regained : that is, the animal may be 

 able to live well enough in some circumstances with- 

 out the aid of the algal cells, while it is certain that 

 the latter may be able to live as independent 

 organisms in the water, procuring their own proteid 

 food as well as their carbohydrate. Alcyonaria, 

 which are colonial animals, are often coloured green 

 in tropical seas, but they are colourless in British 

 waters. Corals in warm seas are often also coloured 

 because they contain associated algal cells, and some 

 species of the protozoan Noctiluca which are colour- 

 less in British seas are green in the tropical waters. 

 We have seen that the tropical seas are poor in 

 plankton, and it is probably the difficulty of procuring 

 sufficient carbohydrate food that has induced the 

 association of chlorophyll-containing algae with the 

 animal organism, for whenever this partnership is set 

 up the difficulty disappears because of the abundance 

 of carbohydrate resulting from the photo-synthetic 

 activity of the algae. 



In all these cases which we have considered the 

 association of the two organisms establishes a com- 

 bination of the two modes of nutrition the holophy tic 

 and holozoic and this is termed heterotrophic. 

 There is no doubt in such cases that we have to deal 



