196 G. H. PARKER 



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

 the esophagus, may reverse the direction of their stroke so that 

 the usual outward current can be converted into an inward one. 

 This reversal is under ordinary circumstances a local response 

 on the part of the cilia to certain dissolved substances in the 

 food. Its relative independence of the other activities of Me- 

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

 is a response to food, excessive feeding has no marked influence 

 on it. Allabach ('05, p. 38) caused a Metridium to gorge itself 

 with food, a process which can result finally in disgorgement, 

 and yet immediately after the animal had emptied itself, its 

 oral cilia were found to reverse to food, which was thus passed 

 down its esophagus. My own observations confirm this state- 

 ment. 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 esophagus are 

 discharged at its open pedal end. By this means in the course 

 of an hour or so I have passed through the esophagus of a 

 Metridium many times the amount of food that its body could 

 have contained, and yet the ciliary reversal was as effective 

 after this period of continuous feeding as before. 



Other evidence of the relative independence of the oral cilia 

 as compared with other effectors is well seen in specimens of 

 Metridium that have been narcotized with chloretone, by which 

 all nervous activity is abolished. A piece of food placed upon 

 the tentacles of such an animal calls forth no special response 

 and either remains where it was placed or moves sluggishly off 

 to the periphery of the disc under the action of the tentacular 

 cilia. When, however, such a piece is put on the lips, the cilia 

 reverse and the morsel is gradually carried down the esophagus 

 and discharged into the gas tro vascular cavity. The swallowing 

 is usually not so rapid as in the normal animal for, under this 

 form of narcotization, the transverse muscles of the mesenteries 

 do not respond to the food by opening the esophagus and conse- 

 quently the ciha are obliged not only to transport the morsel 

 but to force it down a partly closed tube. This, however, the}^ 

 are usually able to do and thus quite independent of neuro- 



