182 
THE OCEAN WORLD. 
Islands form the extremity of another group of atolls, and important 
madreporic reefs, which extend towards the south, by the Maldives 
and the Chagos Islands ; they consist of low coral formations, densely 
clothed with cocoa-nut trees. The Maldives, the most southerly 
cluster, include upwards of a thousand islands and reefs ; the Lacca- 
dives, seventeen in number, are of similar origin. The Saya de Mallia 
hank, towards the south-east, constitutes a further group of madre- 
poric islets. Finally, the coast of the Mauritius, of Madagascar, of 
the Seychelles, and even the African continent, from the northern 
extremity of the Mozambique Channel to the bottom of the Bed Sea, 
are studded with numerous reefs of the same nature. They fail, 
however, almost completely, along the coast of the Asiatic continent, 
where, among others, the waters of the Euphrates, the Indus, and the 
Granges, enter the sea, and diversify its inhabitants. The western 
coast of Africa, and the east coast of the American continent, are almost 
entirely destitute of great madreporic reefs, but they abound in the 
Caribbean Seas. In the Gulf of Mexico, where the vast fresh-water 
current of the Mississippi debouches into the sea, they are unknown. 
It is principally on the north coast and upon the eastern flanks of 
the chain of West Indian Islands that the madreporic reefs show 
themselves in these regions. 
The polypes which have produced these vast ranges of islands 
would be set down, at first sight, as the most incapable objects in 
creation for accomplishing it. In the case of the Pennatulidm , the 
skin is coriaceous, strengthened with calcareous particles ; the interior 
is a fibrous net-work containing a transparent jelly in the squares, and 
permeated by a certain number of longitudinal cartilaginous tubes ; the 
soft part is uniformly gelatinous, but the skin is also coriaceous, with 
a great number of calcareous spicula placed parallel to one another, 
adding greatly to its strength and consistency. 
The polypes are placed in this external fleshy crust ; their position 
being marked by an orifice on the surface, distinguished by eight 
star-like rays, which open when the upper portion of the body is forced 
outwards, in which state it resembles a cylindrical bladder or nipjfle 
crowned with a fringe of tentacula, which surround the mouth. 
Under this orifice is the stomach, occupying the centre of the cylinder. 
The space between this stomach and the outer envelope is divided 
