FEEDING REACTIONS IN CORAL POLYPS. 425 



digestive vacuoles possess an acid reaction whereas in the gastric 

 cavity this reaction is neutral (or weakly alkaline). 



During two days the digestive vacuoles keep approximately the 

 same color. After about 48 hours a few vacuoles have acquired 

 a bluish hue. Gradually the number of the blue vacuoles in- 

 creases and after about three days the majority of the vacuoles in 

 the digestive zone of the mesenterial filaments are blue. At last 

 there are only a few red spots left, whilst the great number of blue 

 vacuoles remain till about seven days after the feeding. During 

 the greater part of the time the color is evenly distributed in the 

 comparatively large vacuoles, but towards the end of the alkaline 

 period (on the sixth and seventh day after the feeding) the 

 coloring matter is concentrated to small particles, which gradually 

 disappear from the mesenterial filaments. 



In the digestion of Astrangia we can therefore distinguish two 

 periods: an acid one, lasting for about two days, and an alkaline 

 period during the rest of the time. In the later part of this 

 alkaline period the excretion of the undigestible particles occurs. 

 The changes in the reaction of the digestive vacuoles make it 

 highly probable that the absorption of the food takes place in the 

 alkaline period only. The acid period then is an antiseptic one 

 (cf. Jordan, 1907^), in which noxious microorganisms are killed. 

 The authors who stated that digestion of actinians takes place in 

 an acid medium (cf. Chapeaux, Mesnil) did not extend their 

 experiments for a sufficient time, otherwise they probably would 

 have found that also in actinians the acid reaction is followed by 

 an alkaline one. As compared with protozoans these two periods 

 last a very long time. In Paramecium and Colpidium after 

 feeding there is an acid period of 5! to 70 minutes which is 

 followed by an alkaline period of I to 30 minutes (Nirenstein, 



I905)- 

 One of the arguments for the presence of extracellular digestion 



in actinians results from the experiments of Willem (1892) re- 

 ferred to above. In Astrangia I have employed approximately 

 the same method to demonstrate the evidence for extracellular 

 digestion. I therefore studied the digestion of large copepods by 

 the polyps. The copepods were vitally stained with neutral red 

 after Fischel's method. At various intervals after the feeding I 



