22 CORALS 



The occurrence of zooxanthellae in the tissues of 

 Coelenterates of various kinds living in shallow waters is 

 very widespread, and in some cases where they are present 

 in unusual abundance the digestive organs of the polyps 

 seem to be degenerating.^ 



These facts and other considerations give strong support 

 to the hypothesis that the zooxanthellae play an important 

 part in the physiological processes of the reef corals. 



The nutrition of the zooxanthellae is probably purely 

 holophytic, that is to say, it is a synthetic process carried 

 on by the action of chlorophyll in sunlight. 



In order, therefore, that the action may be most effective, 

 the zooxanthellae should tend to collect in the superficial 

 canals, and they should be as free as possible from shadows 

 cast by surrounding objects. 



This may account for the fact that has been commented 

 on by so many observers, that the coral polyps are usually 

 contracted in the day-time, and also for the fact that when 

 the digestive centres of the polyps are examined they are 

 usually found to be devoid of food. 



The process of nutrition of corals may therefore be 

 continuous but alternating in character. In the day-time 

 the polyps contract so as to give the sunlight free access to 

 the zooxanthellae in the superficial canals, and in the night, 

 when chlorophyll action must cease, the polyps expand, 

 spread out their tentacles, and capture the animal food 

 which is as necessary for their sustenance as the starch that 

 is passed on to them by the zooxanthellae. 



Whether this is the whole story of the nutrition of corals 

 it is difficult to say. That branch of science which deals 

 with the physiology of the lower animals is still in its infancy, 

 and it may be that in the light of new in\-estigations our 

 views on the nutrition of corals may be profoundly modified ; 

 still it is difficult to believe that the elaborate and highly 

 differentiated organs that the coelenterate polyps possess 

 does not indicate that animal food is an important, if not 

 essential, part of their nourishment. 



1 Edith Pratt, Quart. Joiini. Micr. Sci. 49, 1905. 



