96 GEOLOGY: T. W, VAUGHAN 
short and stumpy or the colony may be ahnost massive in growth form, 
while in quiet water the branches may be decidedly elongated. 
The depth to which the more massive forms extend is between 18 and 
31 meters, 18 meters is usually about the maximum for vigorous growth, 
but some of the branching species extend to slightly greater depths. 
In general the lower depth of the shoal-water coral fauna of the West 
Indies is about 37 meters, approximating conditions in the Pacific. The 
precise cause of the limit in depth has not been determined. Each of 
several possible factors will be discussed. 
All the corals with which I have experimented possess the capacity 
of removing a certain amount of sediment from their surfaces. This 
is affected by the nonnutrient particles becoming imbedded in mucus and 
by cilia removing the mucus and the particles from the surface of the 
tissue. The capacity for cleaning their surfaces varies according to the 
species, it being lowest among those corals which are most important 
on the outer reefs — it is low in Orhicella annularia and high in Maeandra 
areolata. Some corals, as Siderastrea radians, can endure having their 
surfaces covered with silt for some time. This coral seems to secrete 
a layer of mucus which lifts the silt above the tissue surfaces and thereby 
protects them. The branching form of many corals prevents sediment 
settling on them faster than it can be removed. However, as any coral 
will be killed by actual burial beneath sediment, corals can not live where 
sedimentation is rapid; and as sediment accumulates in areas deeper 
than the base of strong wave action or where currents are weak, it is a 
factor in hmiting the depth to which the littoral fauna can extend. 
The mechanism of corals for catching food are as follows: (1) The 
ectodermal surface is beset with nematocysts, which occur on the ten- 
tacles, the oral disk, the column wall, including its downward extension 
called the edge-zone, and also on the margin of the mesenterial filaments. 
(2) The entire ectodermal surface is ciliate, the ciHa in response to cer- 
tain stimuli beating toward the oral apertures; in response to others, 
beating toward the periphery. (3) The outer surface secretes mucus in 
which particles may be embedded, the mucus moving under the influence 
of the beat of the ciHa toward the oral apertures or toward the periphery, 
according to the nature of the response to the stimulation. (4) The 
tentacles are active and effective in capturing food. (5) The mesenterial 
filaments, which in many species of corals can be extruded through the 
column walls, in some instances capture food. 
Many different kinds of food were offered corals, but they took only 
animal food; they are entirely carnivorous. The following experiment 
was tried man}^ times: A piece of diatom mat was placed on one side 
