74 COELENTERATA— CNIDARIA phylum ii 



Neuchatel, and Farringdon, Berkshire ; and also the Middle Cretaceous (Ceno- 

 manian) of Essen, Le Mans, and Havre, are characterised by an abundance of 

 well-preserved Pharetrones, and a lesser number of Sphinctozooid Sycons. In 

 the Tertiary, however, both groups are wanting, although the existence of 

 calcareous sponges is still indicated by occasional detached triactins. The 

 Pharetrones apparently become extinct at the close of the Cretaceous. 



SuBPHYLUM II. Cnidaria. 



The Cnidaria or Nematophora have a radially symmetrical body, and a 

 terminal mouth -opening suiTOunded by fleshy tentacles. In the ectoderm 

 (sometimes also in the entoderm) cnidoblasts are common, from the contents 

 of which thread-cells (nematoci/sts) filled with an urticating fluid and containing 

 a hollow, spirally coiled thread, are developed. Each cnidoblast possesses a 

 fine superficial process (cnidocil), which is very sensitive to mechanical stimuli. 

 The polyp wall typically consists of three layers : an outer ectoderm, an inner 

 endoderm, and a middle mesogloea. The mesogloea is sometimes entirely 

 absent, but the ectoderm and entoderm are strongly developed. The ectoderm 

 frequently secretes a calcareous or horny skeleton, and both ectoderm and 

 entoderm are concerned in the production of muscles and nerves. The sexual 

 organs are the product of the entoderm. 



The Cnidaria are divided into two classes : Anthozoa and Hydrozoa. The 

 latter are undoubtedly the more primitive group, but it will be convenient to 

 treat of the Anthozoa first in the present work. 



Class 1. ANTHOZOA = ACTINOZO A. Coral Polyps.^ 



Usually sessile, cylindrical polyps, possessing a mouth surrounded hy tentacles, 

 oesophagus, and gastrovascular cavity. The latter is divided by numerous vertical 

 partitions (mesenteric folds) into a system of radially disposed pouches. A calcareous 

 or horny skeleton is freqitently developed. Simple or forming colonies. 



The simple polyp zooids have the form of a cylindrical or conical tube 

 at the distal end of which is situated a muscular disk perforated centrally by 



^ Literature : Milne Edioards, H., et Ilaime, J., Histoire naturelle des coralliaires, 3 vols, and 

 atlas. Paris, 1857-60. — Idem, Monograplue des polypiers fossiles des terrains paleozoiques. Arch, du 

 Museum, Paris, vol. v., 1851. — Idem, Monograph of the British Fossil Corals. Palaeontogr. Soc, 

 1849-64. — Fromentel, E. de. Introduction a I'^tude des polypiers fossiles. Paris, 1858-61. — Idem, 

 Paleoutologie frangaise ; 1861 and later. — Reuss, A. E., Articles in Sitzber. Akad. Wiss. Wien, 1859, 

 1864, 1865, 1870 ; also Denkschr. vols, vii., xxiii., xxviii., xxix., xxxi., xxxiii. — Duncan, P. M., British 

 Fossil Corals, 2d ser. Palaeontogr. Soc. 1865-69, and 1872. — Idem, Revision of the Families and 

 Genera of the Sclerodermic Zoantharia or Madreporaria. Journ. Linn. Soc. Zoology, 1885, vol. 

 xviii. — Kohy, F., Monographic des polypiers jurassiques de la Suisse. Abhandl. Schweiz. Pal. Ges., 

 1880-94, vols. vii. -xxii. — Pratz, E., Ueber Septalstructur. Palaeontogr. 1882, vol. xxix. — Koch, 

 G. von. Die ungeschlechtliche Verniehrung der palaozoischen Korallen. — Ibid., 1883, vol. xxix. — 

 Quenstedi, F. A., Petrefactenkunde Deutsclilauds, 1889, vol. vii. — Koby, F., Monographic des poly- 

 piers cretaces de la Suisse. Alihandl. Schweiz. Pal. Ges. 1896-98, vols, xxii.-xxiv. — Ogiivie- Gordon, 

 Maria M., Korallen der Stramberger Schichten. Palaeoutographica, Supp. II., 1897. — Idem, 

 Systematic Study of Madreporarian Types of Corals. Phil. Trans. Roy. Soc. London, 1897, ser. 

 B, vol. clxxxvii. — Gregory, J. W., 1'he Corals, Jurassic Fauna of Cutch. Palaeontol. Indica, 1900, 

 ser. 2, vol. ix., pt. 2. — Vaughan, T. Wayland, Eocene and Lower Oligoceue Coral J^'aunas of the 

 United States. Mon. xxxix, U.S. Geol. Survey, 1900. — Idem, Critical Review of the Literature on 

 •the simple Genera of the Madreporaria Fuiigi<la. Proc. U.S. Nat. Mus., 1905, vol. xxviii. — 

 Duerden, J. E., West Indian Madreporai'ian polyi^s. Mem. Nat. Acad., 1902, vol. \\\\.—Idem, 



