FEEDING REACTIONS IN CORAL POLYPS. 417 



time after feeding air-bubbles are to be seen in the upper parts of 

 the polyps, probably escaping from the prey which is being 

 dissolved in the gastric cavity. 



The reactions of hungry polyps of Astrangia to comparatively 

 heavy objects, as diminutive pebbles and clumps of litmus, 

 writing chalk or iron carbonate, are almost the same. These 

 objects, when laid against a tentacle or on the oral disk, bring 

 about the feeding reaction, consisting of the above described 

 movements of the tentacles and the central part of the oral disk. 

 The objects are almost invariably infested and come closely in 

 contact with the mesenterial filaments. The mouth closes after 

 the ingestion and the polyp remains for some time, in my experi- 

 ments varying between 20 and 95 minutes (usually lasting about 

 50 minutes), quite motionless. The first movement made after 

 this time usually is the enlargement of the central part of the oral 

 disk to a conical protusion. Then the mouth opens and the 

 foreign object is seen moving slowly upwards in the gastric cavity. 

 The underside of the object is in touch with the mesenterial 

 filaments of the polyp, which seem to push against it. In the 

 stomodaeum in all probability the outward movement of the 

 foreign object is caused by ciliary action, but often some mesen- 

 terial filaments are seen protruding in the stomadaeum till the 

 object has been removed from the inside of the polyp. In this 

 way the non-nutrient paticles are soon out of the gastric cavity, 

 chiefly by the action of the mesenterial filaments. The object 

 falls from the mouth on the oral disk and slides down on one side. 

 Here the tentacles bend downwards and then the object is com- 

 pletely removed from the polyp. 



The foreign objects which have been in the gastric cavity for 

 some time are covered with a thin mucous layer. Undoubtedly 

 the mucus has here a protective function, to avoid that noxious 

 particles come into contact with the tissues of the polyp. This 

 function of the mucus is already described by Gee (1913) in 

 Cribrina. In Astrangia especially on the clumps of chalk which 

 are removed from the gastric cavity the mucus is clearly visible. 

 Often other particles imbedded in this layer are removed with the 

 non-nutrient object. In this way very small quantities of re- 

 mains of food may be obtained from the gastric cavity without 



