140 THE ELEMENTARY NERVOUS SYSTEM 



are essential to the acquisition of food ; the production of 

 mucus is apparently a strictly local response to a local 

 stimulus ; the beat of the tentacular cilia is constant and 

 irreversible ; and the opening of the oesophagus is as sim- 

 ple and mechanical a reflex as can well be imagined. The 

 idea that the oesophagus, as often intimated, exhibits per- 

 istalsis is probably incorrect. At least a careful inspec- 

 tion of this organ in action in Metridium gives no support 

 to this idea. The two remaining events in the appro- 

 priation of food, the responses of the oral cilia and the 

 movements of the tentacles, are both open to significant 

 changes and are of the utmost importance in judging of 

 the relation of this process to the actinian as a whole. 



Unlike the tentacular cilia, the oral cilia, those of the 

 lips and of the 03sophagus, may reverse the direction of 

 their stroke so that the usual outward current can be con- 

 verted into an inward one. This reversal is, under ordi- 

 nary circumstances, a local response on the part of the 

 cilia to certain dissolved substances in the food. Its rela- 

 tive independence of the other activities of Metridium 

 can be shown in a number of ways. Thus, though it is a 

 response to food, excessive feeding has no marked influ- 

 ence on it. Allabach (1905) caused a Metridium to gorge 

 itself with food, a process that can result finally in dis- 

 gorgement, and yet immediately after the animal had 

 emptied itself, its oral cilia were found to reverse to food, 

 which was thus passed down its oesophagus. Further, if 

 pieces of meat are fed to the lips of the oral half of a 

 Metridium cut transversely in two, the cilia reverse and 

 the masses of food thus carried through the oesophagus 

 are discharged at its open pedal end. By this means in 

 the course of an hour or so many times the amount of food 



