The Anthozoa are subdivided into three orders, which are based on the peculiar characters of the polypes; the _Hydra_, the _Actinia_ (Sea-Anemone, _Wond._ p. 622), and the _Alcyonium_ (Dead-men's fingers, _Wond._ pl. v. _fig._ 10), being respectively the type of 1. the _Hydroida_, or Hydraform; 2. the _Asteroida_ or Alcyonian; and 3. the _Helianthoida_, or Actiniform zoophytes. In the Hydroida the body in the compound species is implanted in a horny tubular sheath, and the polypidoms form branched corallines, which are fixed by the base to rocks, sea-weeds, shells, &c. The Asteroida have a horny or calcareous axis, surrounded and inclosed by the soft parts which secrete it. The Helianthoids, except in the simple free species, as the Actinia, have a lamellated calcareous polypidom, the plates of which radiate from a centre. 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. [Sidenote: GRAPTOLITES.] [Illustration: Lign. 85. Graptolites in Wenlock Limestone. (_Murch. Sil. Syst._) Fig. 1.--Graptolites ludensis.[233] 1_a_.--Magnified view of a portion of the same. 2.--Graptolites Murchisoni. 2_a_.--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.