GEOLOGY: T. W. VAUGHAN 
99 
being badly damaged, but not entirely killed. Although not precisely 
true, in general the ability to withstand atmospheric exposure is a func- 
tion of the porosity of the skeleton, the species with the more porous, 
surviving longer than those with the denser skeletons. 
The conditions necessary for vigorous coral reef development may be 
summarized as follows: (1) Depth of water, maximum, about 45 meters; 
(2) bottom firm or rocky, without silty deposits; (3) water circulating, 
at times strongly agitated; (4) an abundant supply of small animal 
plankton; (5) strong light; (6) temperature, annual minimum not below 
18°C.; (7) salinity between about 27% and about 38%. 
In the experiments on rearing corals, the planulae were removed with 
a pipette from the vessel containing the parent colony to a jar on the 
bottom of which was a terra-cotta disk. Although the planulae will 
live a long time, even settle in stale water, kept at the proper salinity, 
it is better to change the water at least once a day. To change the 
water, siphons were used, a fine-mesh bolting cloth bag having been 
tied on the end within the jar, so as to prevent the escape of the planulae ; 
while clean water was added through siphons from jars placed at a higher 
level. 
Becaus : of its bearing on the possibility of the distribution of coral 
species by oceanic currents, it is highly important to know the duration 
of the free-swimming larval stage. Observations were made on four 
species. The range was from 2 to 23 days. Should an ocean current 
have a velocity of 3 knots per hour, in 23 days planulae might be carried 
1656 knots; at 2 knots per hour, 1104 knots; at 1 knot per hour, 552 
knots. It is known that every species of shoal water coral in the Ber- 
mudas is found in Florida and the West Indies; while not only is the 
Hawaiian fauna Indo-Pacific in its facies, but several of the species (at 
least 4) also occur on the east coast of Africa or in the Red Sea. I 
seriously doubt any part of the Hawaiian fauna being peculiar to those 
Islands. The clue to the cause of the wide distribution of living coral 
species is given by the possibly long duration of the free-swimming larval 
stage. 
The growth rate of corals was determined by planting planulae which 
attached themselves in the laboratory, by measuring colonies, from 
planulae which settled on collectors in a known season, by measuring 
colonies cemented to disks and fixed on the heads of stakes driven into 
the sea-bottom, and by measuring colonies naturally attached. The 
plantings around the Tortugas were made on the reef oft' Loggerhead 
Key and on the outside of Fort Jefferson moat wall, while records were 
made on colonies growing naturally attached at the two stations men- 
