432 H. BOSCHMA. 



digestive zone have lost their yellow color to some extent. White 

 spots then appear in their interior, whilst the contents are con- 

 tracted and often have assumed a brownish hue. All different 

 stages of the decomposition of these algae are found in the di- 

 gestive region, indicating the probability that they are being 

 digested here. In the polyps of Astrangia which are not infected 

 with zooxanthellae these algae never occur in the digestive region 

 of the mesenterial filaments. Consequently the zooxanthellae do 

 not form a necessary part in the feeding of the polyps. A priori 

 it is highly probable that they are digested on account of the lack 

 of other food. We may therefore expect that, when food of other 

 origin in great abundance is given to the polyps, the cells of the 

 digestive zone of the mesenterial filaments will ingest no more 

 zooxanthellae. 



In fact these changes of the contends of the digestive region are 

 easily accomplished. For my experiments I used dark brown 

 polyps, the digestive organs of which before the artificial feeding 

 contained a large quantity of partially decayed zooxanthellae. 

 These polyps now were repeatedly fed with crab meat and already 

 after three days the digestive region of the mesenterial filaments 

 was almost completely devoid of zooxanthellae. Still a few 

 yellow-brown corpuscles were recognizable as strongly decayed 

 algae, but it was evident that after the feeding of the polyps with 

 crab meat no more algae were ingested. The zooxanthellae which 

 were already present in the digestive organs were completely 

 digested. In this way the polyps can be compelled to change the 

 nature of their food. It is obvious from these facts that the 

 zooxanthellae are ingested by comparatively hungry polyps only. 

 In the natural state the amount of food available for the polyps 

 seems to be rather scanty and therefore they usually derive a part 

 of their food from the digested unicellular algae. 



Van Triet (1919) has described a similar phenomenon in fresh- 

 water sponges (Spongilla), which contain zoochlorellae. These 

 unicellular alga?, which are continually imported from the sur- 

 rounding water, constitute a very important source of food for the 

 sponges. When circumstances are favorable the algae are killed 

 and digested by the sponge only in part, the rest of the imported 

 algae then can live on, photosynthesize and multiply in the tissues. 



