FEEDING REACTIONS IN CORAL POLYPS. 419 



DIGESTION. 



In a great number of anthozoans, chiefly in actinians, the 

 phenomena of digestion are at least partially investigated. The 

 different authors who have worked on this subject do not agree in 

 every detail, though the chief features of the digestion are fairly 

 well known. 



The principles of our knowledge of the phenomena of digestion 

 in actinians and other ccelenterates are chiefly due to Metschnikoff 

 and Krukenberg. Metschnikoff (1880, 1882) found that small 

 food-particles are ingested by the entoderm cells of the mesen- 

 terial filaments in an amoeboid way. They are imbedded in these 

 cells and digested here (intracellular digestion). Claus (1881) 

 maintained that the ingestion of foreign corpuscles in the mesen- 

 terial filaments of ccelenterates was already described by him 

 before Metschnikoff made mention of this fact. But in Met- 

 schnikoff's papers the phenomenon of ingestion was described in 

 quite a convincing form, whilst Claus' previous investigations 

 (Claus, 1874) only contained some notes on the question. 



Krukenberg (1880) proved that the mesenterial filaments of 

 actinians are organs of digestion and maintained that digestion 

 takes place solely against these organs. According to this author 

 the mesenterial filaments have to come in contact with the food, 

 then a secretion of digestive fluid takes place and the food- 

 particles are more or less dissolved (extracellular digestion) 3 . 

 Krukenberg (1881, 18820, 1882-86) was not inclined to attribute 

 such an important function to the intracellular digestion of small 

 particles as described by Metschnikoff. He was convinced that 

 the filaments secrete a digestive fluid, though he was not able to 

 demonstrate this fluid in a free state in the gastric cavity. He 

 even expressed the opinion that the enzymes of the captured 

 animals could be used by the actinians for their digestion (Kru- 

 kenberg 18826, i882c). 



The investigations of Willem (1892, 1893) made it probably 

 that besides intracellular digestion also extracellular digestion of 



3 In the work of O. and R. Hertwig (1879/80) already the opinion is put forward 

 that the mesenterial filaments secrete a digestive fluid. This was based only on the 

 study of the histological structure of these organs. The definite proof of the diges- 

 tive function of the mesenterial filaments is given by the physiological studies of 

 Metschnikoff and Krukenberg. 



