The reef-building corals comprise all those with a stony skeleton, yet the great work is carried on mainly by a few of the principal groups. Of these the following are the most conspicuous and familiar:
The Astrœas, so called from their star-shaped cells (Fig. 6, page 261). They grow in huge hemispherical masses, often twenty feet or more in diameter. The brain-coral, covered with meandering furrows and ridges resembling cerebral convolutions (Fig. 1, page 282). The masses are large, and are shaped like the astræa. The Porites, often branched,
Fig. 9.
Millipores; Coral secreted by Jelly-fish (Acalephs).
sometimes massive and covered with exceedingly minute cells, are represented in Fig. 8, page 263. Other species are branching or lamellar, as the Millipores (Fig. 9), which contribute largely to the material of the West-India reefs. The animal, however, in this case, is not a true polype, but belongs to the group of acalephs, or jelly-fishes. Then there are the beautifully branched, tree-like Madrepores (Fig. 10, page 205). Fig. 11, page 266, represents one of the most beautiful of the corals produced by the alcyonoid polypes. Almost all of these are flexible, and sway with the moving waters. Some kinds are too flexible to stand erect, and they hang from the coral ledges, as in the coral caves, in gorgeous clusters of scarlet, yellow, and crimson. All these corals are covered with cells, and each represents the dwelling
