THE ROLE OF MUOUS IN CORALS. 609 
is from time to time separated in irregular patches from the 
general surface, and wafted away over the edge of the disc 
by the excurrent stream from the polypal cavity. In these 
movements over the discal surface the patches and strands of 
mucus may become more or less rolled into boli. Parker also 
found in Metridium that powdered carmine in sea-water 
when dropped upon the tentacles was matted together in 
threads as though by mucus, and was slowly carried from the 
bases of the tentacles towards their tips, but he nowhere 
attaches much significance to the secretion. 
After removal of the impregnated mucus the polypal surface 
is fresh and clean, and there can be no question that one of 
its functions is thus to protect and keep clean the walls of 
the polyp, there being no other means by which this can be 
accomplished. Light bodies falling upon the polyp, whether 
nutritive or otherwise, do not come into direct contact with 
the ectodermal cells but are received in a thin mucous layer, 
which is shed from time to time and got rid of by means of 
currents from the stomodeum, aided perhaps by the move- 
ments of the polypal walls and tentacles. 
On the column of many actinians, e.g. Phellia, Adamsia, 
the mucus is not regularly removed, as it is in corals, but 
accumulates and hardens somewhat so as to form a rather 
thick membrane, having sand grains, foraminifera, diatom 
frustrules, and other foreign bodies embedded in it. In these 
cases the mucus appears as a rough protective membranous 
coating close to the polypal wall, and is shed only at wide 
intervals, after which the column appears thinner walled and 
more delicate. The upper part of the column (capitulum) 
and discs of the polyps remains permanently smooth, the 
mucus and any adhering foreign particles being regularly 
removed from this region. Likewise the tube in Cerian- 
thus is largely formed of exuded slime, in which nemato- 
cysts and foreign particles have become embedded. Loeb? 
has shown that the secretion of the tube of Cerianthus. is 
1 Loeb, J., “Studies in General Physiology,” 1905, p. 165, 
