FEEDING REACTIONS IN CORAL POLYPS. 43! 



and digestive portions of the polyps has taken place, as in Sclero- 

 phytum gardineri (Pratt, 1903, 1906) and in Galaxea musicalis 

 (Matthai, 1914). 



The feeding of coelenterates on their symbiotic algse as a whole 

 has been reported in a few cases only. In young medusae of 

 Aurelia aurita Friedemann (1902) has mentioned instances of this 

 phenomenon. According to Pratt (1903) in the polyps of the 

 alcyonarian coral Sclerophytum frequently zooxanthellae are ob- 

 served in a partially digested condition in the mesenterial fila- 

 ments (cf. also Pratt, 1906). Digestion of zooxanthellae in the 

 polyps of reef corals is recorded by myself (1924). Probably the 

 irregular green corpuscles which McMurrich (1889) found among 

 the zooxanthellae in the digestive part of the mesenterial filaments 

 of zoanthids were also such partially digested algae. In Velella 

 the zooxanthellae of the developing larvae which migrate to deeper 

 water probably serve as a source of food-substance only, as their 

 photosynthetic action is impeded by the darkness (Woltereck, 

 1904). 



According to Fulton (1921, 1922) the association of coelente- 

 rates with their zooxanthellae probably is of a parasitic nature (the 

 polyps being the parasites of the algae), as during starvation sea- 

 anemones feed upon the unicellular algae rather than upon their 

 photosynthetic products. 



Besides their feeding on the zooxanthellae or on their products 

 of assimilation the polyps also derive some profit from the algae 

 living in their tissues, as they are a source of oxygen for the polyps. 

 The investigations of Brandt (1883), Trendelenburg (1908) and 

 Putter (1911) prove that actinians with zooxanthellae may derive 

 a large part of their oxygen from these algae, and that actinians 

 with zooxanthellae can better resist unfavorable circumstances 

 than those which do not harbor unicellular algae. Probably the 

 algae also use nitrogenous waste products of the polyps, which may 

 be one of the profits the zooxanthellae derive from the association 

 with the coelenterates. 



In the case of Astrangia zooxanthellae are always found in the 

 digestive region of the mesenterial filaments of the polyps which 

 are infected with these algae. In contradistinction to the algae in 

 the entoderm of the oral disk and the tentacles those in the 



