THE ROLE OF MUCUS IN CORALS. 601 
are absent in all coral polyps the conditions governing 
the inhalent and exhalent currents are simpler in corals than 
in actinians. In the latter the water expelled over the 
general surface of the stomodeum can be replaced synchro- 
nously by that flowing down the siphonoglyph, whereas in 
corals the two streams must be intermittent. Hence the 
greater probability that in corals, at any rate, inhalent streams 
may be established without the influence of external stimuli, 
and carry with them whatever foreign particles may be upon 
the disc. 
The actual conditions determining the reversal of the cilia 
have been studied by Parker'. He shows both by experi- 
mental means and by actual observation that the labial cilia 
do not reverse when in contact with carmine, Indian ink, 
sand, pellets of filter-paper moistened with sea-water, or with 
solutions of sugar, quinine, or picric acid in sea-water, but 
that they reverse to dilute crab-meat juice. Jurther, the 
reversal occurs only over the area where the stimulus is 
applied, as was found to be the case in Fungia. Investigating 
the subject chemically he finds that the ciliary reversal due to 
crab-meat is probably dependent upon some organic combina- 
tion containing potassium, for the cilia reverse in a 2} per 
cent. solution of potassic chloride in sea-water, though not 
in a large number of other chemical substances tested ; 
reversal is found to be due to potassium ions, since it occurs 
in 2m NaCl + +m KNO,, but not in 3m NaCl+imNa 
NO. 
Parker has fully demonstrated that the tentacles of Metri- 
dium are ciliated, but here the direction of the stroke is not 
reversed by the application of meat juices; likewise the 
inward beat of the cilia lining the siphonoglyph is non- 
reversible. Any part or all of the rest of the stomodzal 
cilia, however, reacts to the influence of nutritive juices. 
aleyonarian, Aleyonium digitatum, tbe cilia of the single siphonoglyph 
produce a current of water from without inwards, whereas the cilia lining the 
rest of the stomodzum produce currents in the opposite direction, 
1 ¢ Amer, Journ, Physiol.,’ vol. xiii, 1905. 
