t half a year,[1115] so that the rate of
increase may, under favorable circumstances, be very far from slow.

It must not be supposed that the calcareous masses termed coral reefs
are exclusively the work of zoophytes: a great variety of shells, and,
among them, some of the largest and heaviest of known species,
contribute to augment the mass. In the South Pacific, great beds of
oysters, mussels, _Pinnae marinae_, _Chamoe_ (or _Tridacnae_), and other
shells, cover in profusion almost every reef; and on the beach of coral
islands are seen the shells of echini and broken fragments of
crustaceous animals. Large shoals of fish are also discernible through
the clear blue water, and their teeth and hard palates cannot fail to be
often preserved although their soft cartilaginous bones may decay.

It was the opinion of the German naturalist Forster, in 1780, after his
voyage round the world with Captain Cook, that coral animals had the
power of building up steep and almost perpendicular walls from great
depths in the sea, a notion afterwards adopted by Captain Flinders and
others; but it is now very generally believed that these zoophytes
cannot live in water of great depths.

Mr. Darwin has come to the conclusion, that those species which are most
effective in the construction of reefs, rarely flourish at a greater
depth than 20 fathoms, or 120 feet. In some lagoons, however, where the
water is but little agitated, there are, according to Kotzebue, beds of
living coral in 25 fathoms of water, or 150 feet; but these may perhaps
have begun to live in shallower water, and may have been carried
downwards by the subsidence of the reef. There are also various species
of zoophytes, and among them some which are provided with calcareous as
well as horny stems, which live in much deeper water, even in some cases
to a depth of 180 fathoms; but these do not appear to give origin to
stony reefs.

There is every variety of form in coral reefs, but the most remarkable
and numerous in the Pacific consist of circular or oval strips of dry
land, enclosing a shallow lake or lagoon of still water, in which
zoophytes and mollusca abound. These annular reefs just raise themselves
above the level of the sea, and are surrounded by a deep and often
unfathomable ocean.

In the annexed cut (fig. 114), one of these circular islands is
represented, just rising above the waves, covered with the cocoa-nut and
other trees, and inclosing within a lagoon