GEOLOGY: T. W. VAUGHAN 
97 
of the oral disk and a piece of crab meat on the other. Invariably the 
crab meat was seized and swallowed; while the diatoms induced no 
reaction except ultimately to be removed from the surface. No kind 
of purely vegetable food was taken by any one of the numerous species 
investigated. However, pieces of plants coated with small animals 
or soaked in meat juice will be swallowed, and later the vegetal matter 
ejected. 
As the food of corals is purely animal plankton, a decrease in the 
amount of this food-supply with increasing depth, would limit the down- 
ward distribution of the shoal water forms, but as I do not know of any 
quantitative estimates of the amount of animal plankton above and 
below 20 fathoms in coral reef areas, there is no basis for a positive 
opinion. 
The relation of corals to light was studied. Specimens of 17 species 
were put into a darkened, light-proof live-car. One of the number was 
dead at the end of 14 days; 3 others were dead at the end of 28 days; 
while 11 species survived at the end of 43 days. However, all had 
become pale, some even colorless, or otherwise showed abnormalities. 
A natural experiment, which appears conclusive, is afforded by Fort 
Jefferson wharf. Here corals thrive on all the outer piers where the 
light is strong but there are none on the central piers where there is per- 
petual shade. It therefore seems to me that strong light is essential 
for the vigorous growth of shoal water corals. 
Another factor is temperature. Dr. Mayer conducted a series of 
experiments to ascertain the higher and lower limits of temperature 
which the common corals around the Tortugas can endure. These 
indicate that a lowering of the temperature to 13.9°C. would extermin- 
ate the principal Florida reef corals, while the most important inner 
flat corals would survive. He obtained similar results on the corals 
around Murray Island, Australia. 
Dr. H. F. Moore of the U. S. Bureau of Fisheries has communicated 
to me temperature records made at lighthouses along the Florida reef. 
These show that vigorous reefs v/ill endure a temperature as low as 18.15° 
C, the minimum at Carysfort light between 1879 and 1899; but at 
Fowey Rock, where the minimum drops to 15.6°C. although there are 
some corals, there is no thriving reef. The species found at the north 
end of the reef line are those which Dr. Mayer's experiments showed 
capable of withstanding the lowest temperature. The temperature 
records for the reef line indicate 18.15°C. as the minimum temperature 
which a reef v/ill survive — this is 1.85°C. lower than the figure given by 
Dana. It is not probable that a reef could withstand a continuous 
