The earliest Corals correspond in certain features of their structure to the Millepores.  They differ from them as all early animals differ from the succeeding ones, every geological period having its special set of representatives.  But still they are always true to their class, and have a certain general correspondence with animals of like kind that follow them in later periods.  In this sense the Millepores are in our epoch the representatives of those early Corals called by naturalists Tabulata and Rugosa,—­distinguished from the Polyp Corals by the horizontal floors, waving in some, straight in others, which divide the body transversely at successive heights through its whole length, and also by the absence of the vertical partitions, extending from top to bottom of each animal, so characteristic of the true Polyps.  As I have said, they were for a long time supposed, notwithstanding these differences, to be Polyps, and I had shared in this opinion, till, during the winter of 1857, while pursuing my investigations on the Coral Reefs of Florida, one of these Millepores revealed itself to me in its true character of Acaleph.  It is by its soft parts alone—­those parts which are seen only in its living state, and when the animal is fully open—­that its Acalephian character can be perceived, and this accounts for its being so long accepted as a Polyp, when studied in the dry Coral stock.  Nothing could exceed my astonishment when for the first time I saw such an animal fully expanded, and found it to be a true Acaleph.  It is exceedingly difficult to obtain a view of them in this state, for, at any approach, they draw themselves in, and remain closed to all investigation.  Only once, for a short hour, I had this opportunity; during that time one of these little creatures revealed to me its whole structure, as if to tell me, once for all, the story of its existence through all the successive epochs from the dawn of Creation till now, and then withdrew.  With my most patient watching, I have never been able to see one of them open again.  But to establish the fact that one of the Corals represented from the earliest period till now, and indeed far more numerous in the beginning than any other, was in truth no Polyp, but an Acaleph, the glimpse I had was all-sufficient.  It came out as if to bear witness of its class,—­as if to say, “We, too, were among the hosts of living beings with which God first peopled His earth.”

With these branching Corals the reef reaches the level of high-water, beyond which, as I have said, there can be no further growth, for want of the action of the fresh sea-water.  This dependence upon the vivifying influence of the sea accounts for one unfailing feature in the Coral walls.  They are always abrupt and steep on the seaward side, but have a gentle slope towards the land.  This is accounted for by the circumstance that the Corals on the outer side of the reef are in immediate contact with the pure ocean-water,