114 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
to a chemical stimulation of the lips or gullet, or both. The inward 
gliding of the piece of meat over the lips is due to ciliary action, but, as 
the cilia of the lips usually wave outward, this movement can be ex- 
plained only by assuming that the cilia have reversed their stroke. For 
a metazoan this certainly is very exceptional, and I was at first very 
much inclined to doubt the truth of this explanation, but my doubts were 
entirely put aside by the following observation. When a portion of the 
lip of a living Metridium is cut out and examined under a microscope, 
the cilia can be seen to wave, as indicated by the many small foreign 
partieles suspended in the fluid about them, in a direction away from the 
side that was next the mouth. If now a quantity of dilute meat juice 
be run in under the cover-glass of such a preparation, the moment the 
juice comes in contact with the cilia they can be seen to reverse their 
movement and wave in the opposite direction. The reversal of the 
cilia is, then, an observed fact. 
The usual stroke of the cilia of the lips, as I have already implied, 
is in a direction away from the mouth; for, when carmine is dropped 
upon the lips, this glides immediately outward. Moreover, in excised 
pieces of the lips examined in pure sea-water under a microscope, the 
same outward movement is obvious. Further, when a small piece of 
meat is dropped on the lips, it does not pass at once inward, but moves 
momentarily outward, and is then reversed and glides into the month. 
These observations show, I believe, that the cilia of tho lips usually wave 
away from the mouth, and reverse their action only in the presence of 
food or similar substances. 
The extent of the area over which the reversal of the cilia takes place 
can be easily demonstrated. If a quantity of carmine be discharged 
over the lips of a, Metridium, the cilia by their usual action will begin 
Sweeping the particles outward. If during this operation a piece of 
meat be dropped on the lips, it will be seen that, though the cilia im- 
mediately around the piece of meat reverse their movement, those in 
front, behind, and at either side of it continue to wave outward, the 
area of reversal growing in front and dying out behind as bhe particle 
of meat glides toward the mouth. In no case was the reversal of the 
cilia on the area of the stimulated lip accompanied by a reversal on other 
parts of the same lip or on the opposite lip. Repeated observations of 
this kind prove that the reversal of the cilia is due to direct local 
stimulation, and lasts only as long as the stimulating body is present. 
There is nothing to favor the view that the ciliary action, even though 
subject to reversal, is under any form of nervous control. 
