of Edinburgh, Session 1885-86. 
863 
coral at the base of the branches, where it lives ; whilst the light- 
red colour of the big claws, as they are held up in their usual 
attitude, similarly imitates the colour of the branches themselves. 
To make the guise more complete, both carapace and claws possess 
rude hexagonal markings which correspond exactly both in size 
and appearance with the polyp-cells of the coral. Another species 
of crab, that climbs about the blue-tipped branches of a Madrepora , 
has the points of its pincer-claws similarly coloured. It is interest- 
ing to note that these two crabs are adapted to live each on its own 
species of coral. Had I caused them to exchange their homes, 
their borrowed hues and markings would have at once made them 
conspicuous objects for their enemies. 
The Inter-Tidal Exposure of Living Corals. — A short exposure to 
the air is generally stated to be fatal to living corals. The state- 
ment of Mr Beete J ukes that an exposure of two or three hours to 
the air, sun, (and rain), will not kill many of the coral polyps as long 
as they are left in their position of growth and thus retain their 
moisture, has not often been quoted, but it nevertheless accords 
with my experience.* 
Different kinds of reef-corals, however* bear different degrees of 
inter-tidal exposure. The minimum amount is to be seen in the 
case of many of the massive and branching species, which at low 
tide, and especially at “low water springs,” are bared for a few 
seconds twice or three times in the minute, as the larger waves roll 
past them. To this group belong species of Porites, Meandrina , 
Seriatopora ( hystrix , D., and pacijica Br.), and Millepora , &c. 
Then, there are corals which can sustain exposure to the extent of 
from 5 to 10 inches for an hour with impunity, as long as they are 
washed over by a wave every three or four minutes. Such corals are 
Heliopora coerulea , Millepora platyphylla (Ehr.), and intricata (E. & 
H.), Stylophora mordax (D.), Pocillopora sp., and several species 
of Madrepora. Lastly, we come to the corals that can withstand 
continuous exposure for from one to two hours without injury. I 
have seen the nobs of Porites tenuis (V.), which are so common on 
the reef-flats, uncovered for nearly three-quarters of an hour. The 
living margin of a boss of a Symphyllia I have seen exposed to the 
extent of 6 inches for about an hour. Some of the Madreporce are 
* Voyage of H.M.S. Fly, 1847, vol. i. p. 119. 
