Page:The New International Encyclopædia 1st ed. v. 05.djvu/468

* COBAL. 402 COBAL. nialian brain or a meadow brook, hence its teeb- nioal name is Jleaudrina. In the greater num- ber of kinds, besides the pbites whicb form and separate the polyp-cells, and wbicb are variously arranged, tbere is a more solid internal or cen- tral part, formed by the additional deposition of matter at the bottom of each polyp-cell, or from the common living part in which the polyps are united. The calcareous framework is sometimes further strengthened by a greater or less mixture of horny aiiimal matter with the purely calcareous substance. Kinds of Vomls. — Corals are roughly classed under two heads, the horny corals and the lime or stone corals. The former consist chiefly of a horny secretion from the polyps, which may in- clude also separate particles of lime, while the stone corals consist almost wholly of lime firmly united in a solid mass. No sharp line can be drawn between these two groups, for all possible gradations can be foimd. The Antipatharia and Alcyonaria as a rule have a horny skeleton, while the millepores and madrepores are almost wholly limestone. The polyps of the common red or precious coral {CoralJiiim riihy-iiin) be- long to the suborder Alcyonaria (q.v., for illus- tration) ; but the central axis in this and other corals forming the family Corallidse is quite solid, and is produced in concentric layers by the living gelatinous substance which envelops it like the bark of a tree, and from which the polyps project like buds, or, when their tentacles are expanded, like little flowers. Another alcyona- rian coral of more than usual interest is the 'organ-pipe' coral (Tubipora) in which no cen- tral axis fills in behind the polj-ps, but the chambers in which they live remain as open, elongated tubes, crowded together side by side to form a solid mass. Its polyps are violet or grass-green, but the coral itself is red or purplish. These corals are found in the Indian and Pacific oceans. jMany of the madrepores have the whole calcareous framework covered, as in the Coral- lida?. by a gelatinous living substance which unites all the polyps. The whole living part soon decomposes and disappears, when the coral is taken out of the water, in some species almost immediately running from the calcareous part as a watery slime. VtiUzntwn. — Red coral — so much admired for its fine color and susceptibility to a high polish, and much used for ornamental purposes — ^is chieflv obtained from the jMediterranean, in some' parts of which extensive 'fisheries' are carried on. It is brought up from consider- able depths by means of a sort of grappling apparatus dragged after a boat or boats, the pieces being broken from the bottom by beams of w^ood which are sunk by weights and then entangled among hemp. Eed coral has a shrub- like branching form and grows to the height of about one foot, with a thickness like that of the little finger. IMuch of the coral of the Mediterranean is exported to India, but red coral is also obtained in the Tied Sea, the Persian Gulf, etc. Black coral ( Antipathes), the axis of which is more solid, is still more highly prized. Coral was known to the ancients, and was used for ornamental purposes by the Gauls. l{,>efs, — The formation of coral reefs and is- lands is one of the most noteworthy results of the action of coral polyps, ilany kinds of polyps are concerned in the building of a reef, but the growth is mainly dependent on the stone corals. Reef-building corals only flourish in dean, fresh sea-water, which is not over 1"25 feet in depth and never is cooler than 68° F. The}- are con- sequently confined to the tropics and to shallow water, and neer thrive near the mouths of rivers. Although reef-building corals are not found on the coast of the United States north of Florida, some species of coral occur much further north. Even on the coast of New England there is to be found quite commonly a true stone coral (Astrnn- gia Danw), the polyps of which when expanded are large and very beautiful. Fossil Forms. Among the fossil corals only those belonging to the Anthozoa are of importance, those of the Hydromedusie either not ap])earing until the Jlesozoic, and then but sparingly (for fossil Hydrocorallime and Tubularis, see C'celex- terata), or not being clearly recognized as to their sj'stematic position and only provisionally referred to the Hydrozoa. ( See Stroiiatopoka and Graptolite.) The Anthozoa appear in the Cam- brian rocks with the Archajoeyathinfe, forms of peculiar structure and uncertain relationships. In the I'pper Silurian period they have become the most important fossils, as well in number of in- dividuals as in diversity of structure and impor- tance as rock-building organisms. They continue thus throughout geologic times, and are hence of great importance to the paleontologist and geolo- gist. As they are also objects of great beauty, they are niucii sought by collectors and dealers. Especially rich and famous are the fossil coral faunas of the Silurian of North America and Got- land; the Devonian of the Helderbergs of New York, the Falls of the Ohio, and the Rhenish provinces; the Jurassic formations of Middle Europe and the Tertiary beds of northern Italy. The Anthozoa are divided by Haeckel into the sub-classes Tetracoralla, Hexacoralla, and Octo- coralla, according to the nimiber of their septa, which were considered to be multiples of these figures. The Tetracoralla comprise, together with the Tabulata, the Paleozoic corals; the Hexa- coralla and Octocoralla, the ]Iesozoic and later forms. Recent investigations on the embryology of the Hexacoralla and thecal structure of the Tetracoralla (Dr. ]M. Ogilvie) seem to demon- strate that these divisions are artificial, that the tetrameral sVstem is only an ancestral feature strongly marked in certain of the old families (Cyathophyllidae, Zaphrentidse. Cyathaxonida"), while hexameral symmetry is but one of many forms of radial symmetry. The Anthozoa are therefore at present divided into but two sub- classes, according to Ray Lankester: the Alcyo- naria or Octocoralla, and the Zoantharia. None of the fossil alcyonarians, which with doubtful forms do not appear till the Mesozoic. are of importance; the Paleozoic HeliolitidiP, which a.l'e referred with doubt to this sub-class, are quite abundant in the Silurian and Devonian rocks. The sub-class Tabulatse, of Milne-Edwards and Hainie, comprising common and important Paleo- zoic genera, such as Favosites, Aulopora. Syringo- pora. Halysites, and Cho'tetes, which was long considered as a distinct sub-class characterized by the slight development of the septa and the presence of munerous tabulae, has now been broken up : it having been recognized that some families belong to the Octocoralla, others (Favositidie, Syringoporidse, Halysitida") exhibit close relationships to the Hexacoralla, and