The only parts which remain soft are the summit, the mouth, the fringe of tentacles about it, and the stomach within. When this first animal has reached its maturity, offspring begin to grow from its sides, by a process which resembles budding, and thus a large colony rapidly spreads itself, all the individuals of which are connected with one another by openings at the base of the body, so that the nourishment imbibed by one circulates through his neighbor's system and contributes to feed the whole colony. On foundations laid in pretty deep water by the solid masses of Astrea , arise the reefs which in circular form make the atols of the Pacific, or in straight banks act as barriers for continental shores. But, when they have brought the wall so high that they have not more than six fathoms of water above them, this sort of coral ceases to grow. The next class is that of the Echinoderms or spiny- skinned animals.
Echini live on rocky shores, and also at great depths.
They form an important part of the food of many coast savages. The ordinary star-fish, well represented by specimens, attains importance, apart from scientific interest, by his ravages upon our oyster beds. No idea can be formed of the beauty of these animals, either from dried specimens or from those preserved in alcohol. Many scientific men of prominence have contributed labor and skill toward perfecting the arrangement and condition for study of this huge collection of the fossils of the lower forms of life, which have formed the basis for the volumes of Profs. F. B. Meek, F. V. Hayden, James Hall, C. A. White, and others in America and Europe.