156 PROCEEDINGS OF THE AMERICAN ACADEMY. 



oral disk fail to bring about muscular reactions, even though the gran- 

 ules come into contact with the tentacles when being carried peripher- 

 ally by the surface cilia. Furthermore, unless the stimulus is a strong 

 one, the edge zone Avill not fold inward far enough to form a complete 

 roof. As has been seen, a piece of filter paper soaked in dilute meat 

 extract is" not a sufficient stimulus to bring about the contraction of the 

 sphincter muscle, although it does cause a sinking of the oral disk, with 

 eversion of the gullet, and partial extrusion of mesenterial filaments 

 through the mouth. But the roofing over of the oral disk is not a nec- 

 essary j)art of the process of extra -coelenteric digestion, provided the 

 food organisms are prevented from escaping by the tentacles or by the 

 mesenterial filaments themselves. 



As may now be seen, the cilia of the oral surface apparently play an 

 unimportant role in the feeding process. Their chief function seems to 

 be that of keeping the oral surface clean. Their abundance, and their 

 persistency in driving foreign particles toward the periphery, may be 

 correlated with the comparatively small amount of surface mucus, which 

 in such polyps as Favia and Fungia forms a well-developed protective 

 layer over the oral disk (Duerden, : 06). 



After the rose-coral polyp has completed the digestion of its food in 

 the supra-discal cavity, the stomodaeum and mesenterial filaments are 

 retracted, and the sphincter and mesenterial muscles relaxed. Water is 

 then drawn in through the mouth by the action of the stomodaeal cilia, 

 and the polyp expands into its resting condition. The cilia of the oral 

 surface now come into play, and transport peripherally any undigested 

 particles that may have been left on the oral disk. Such particles, as 

 may be seen by watching carmine grains, are carried across the edge 

 zone, and deposited in the grooves {IG, Plate) which separate adjacent 

 polyps. 



Feeding Tleactions in Other Corals. In its method of taking food, 

 as above described, the rose-coral zoiiid diff'ers from the polyps of the 

 three other madreporarian genera whose feeding reactions have been 

 observed. Two of these, Favia and Fungia, were studied by Duerden 

 (:06) in the Hawaiian Islands. The oral disks of Favia and Fungia 

 are not ciliated, but are well provided with gland cells, which secrete 

 on the external surface an abundant coating of mucus. In this mucus 

 foreign particles become embedded. Ordinarily the beat of the stomo- 

 daeal cilia is outward, but under the stimulus of meat juice the cilia 

 reverse, and the strong inhalent currents thus produced waft into the 

 mouth shreds and patches of the mucus, together with the foreign ma- 

 terials entangled or embedded in it. The writer evidently considers 

 that these foreign materials furnish the polyp with its food. 



