The calcareous secretions of the Anthozoa, especially of the Helianthoida, in a great measure constitute the mass of the coral-reefs and coral-islands of tropical seas. Their polypidoms, whether external or internal, maintain but little organic connexion with the compound soft substance. These zoophytes increase by gemmation or budding; some throw up germs from the disk, as in Astreadæ; others laterally, as in Caryophillidæ; and some spirally along the stem, as in Madreporidæ; examples of these modes of reproduction are often found in fossil corals. The increase of coral-rocks is produced by the continual formation of new masses, by the successive generations which spring up as it were from the bodies of their parents; layer upon layer, and tier upon tier, of Helianthoid polypidoms, are found to compose many of the coralline limestones of the palæozoic formations.
Fossil Anthozoa.—The first group of extinct corals to be noticed under this head is the Graptolitidæ, a family restricted to the Silurian rocks, and whose natural affinities have been much questioned; some palæontologists referring them to the Pennatulidæ, or Sea-pens, others to the Sertulariadæ.[232]
[232] For a full consideration of this subject, refer to Prof. McCoy's Brit. Palæozoic Fossils.
GRAPTOLITES.
Lign. 85. Graptolites in Wenlock Limestone.
(Murch. Sil. Syst.)
| Fig. | 1.— | Graptolites ludensis.[233] |
| 1a.— | Magnified view of a portion of the same. | |
| 2.— | Graptolites Murchisoni. | |
| 2a.— | Magnified portion of fig. 2. |
[233] Ludensis, from Ludlow—to indicate the habitat of the fossils.
Graptolites. [Lign. 85.]—These curious zoophytes abound in many of the Silurian deposits; they consist of sessile polype cells, arranged in one or two rows to a flexible stem, like the recent Sertularia, or Virgularia. Prof. McCoy refers them to the order Hydroida.