[48]The duller coloured and peculiarly shaped “bladder” or “parrot” fish, Tetraodon or “four tooth” is distinguished by its peculiarly rounded shape and alteration of scales into spines. Though its teeth are very similar to the coral eater, Pseudoscarus, it eats softer animals, e.g. Ascidians and Echinoderms, and sometimes shell-fish.
[49]Includes the whole oyster and bivalve shell-fish family, and many other less generally known forms of life.
[50]Owing to the practical absence of tide in the Red Sea the boat channels of the fringing reef are discontinuous, so that at intervals the canoe men must wait for favourable weather and put out to the open sea. In Zanzibar, e.g., one can travel the whole 60 miles of the east coast within the reef but for the crossing of one bay, except at lowest spring tides. Some of the deeper parts of the boat channel of Red Sea reefs are due to faulting (see next chapter) as may be also a very peculiar reef channel just north of Mombasa Harbour.
[51]For the qualities and formation of this rock see [page 111].
[52]The absence of similar stones from all other parts of the reef is to be accounted for by their being subject to the continual friction of strong sand-laden currents, while those on the raised edge are exposed to waves of clean water for a portion of each day only.
[53]These tubes form a mass by coiling loosely together like a cluster of worms, though the animal which makes them is exactly like those that have regular spiral shells and move about freely, e.g. the whelks. The young Vermetus has a shell like a young whelk, but as it then fixes itself down, the shell degenerates, and grows into the loosely coiled tube of the adult mollusc. Nothing could be more different than the body of a Vermetus and that of a worm, the resemblances between the shell of the one and the tube of the other being pure coincidence.
[54]More correctly a compound, not mixture of carbonates, is formed, viz. dolomite, CaMg(CO₃)₂.
[55]Examples of all three are well shewn on the map opposite [page 136].
[56]I.e. in the great oceans. As shewn later, volcanic mounds and islands are found in the Red Sea, and may have formed the foundations of certain of its reefs.
[57]Gardiner, J. S., “The building of Atolls,” Proc. Camb. Phil. Soc. or The Challenger Society’s Science of the Sea, John Murray.