SUB-BRANCH II CNIDARIA—ANTHOZOA 67 
calcareous sponges is still indicated by occasional detached triactins. With the 
close of the Cretaceous, the Pharetrones seem entirely to have disappeared. ! 
Sup-Brancu Il. Cnidaria. 
The Cnidaria or Nematophora have a radially symmetrical body, and a 
terminal mouth-opening surrounded by fleshy tentacles. In the ectoderm 
(sometimes also in the entoderm) enidoblasts are common, from the contents 
of which thread-cells (nematocysts), 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 mesoderm is sometimes entirely absent, but the ectoderm and entoderm are 
strongly developed. The ectoderm or mesoderm frequently secretes a calcareous 
or horny skeleton, and both ectoderm and entoderm are concerned in the pro- 
duction of muscles and nerves. The sexual organs are the product of the 
entoderm. 
The Cnidaria are divided into two classes: Anthozoa and Hydrozou. 
Class 1. ANTHOZOA=ACTINOZOA. Coral Polyps.2 
Usually_sessile, cylindrical polyps, possessing a mouth surrounded by 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 frequently developed. 
The simple polyp individuals have the form of a cylindrical or conical tube, 
at the distal end of which is situated a muscular disk perforated centrally by 
the sht-like or oval fissure of the mouth. The mouth is furnished with a ring 
of tentacles round its margin, and opens into a membranous oesophageal tube 
1 To the sponges, and in fact to the Hexactinellids, Hinde has referred the singular group 
Receptaculidae occurring in the Ordovician, Silurian, and Devonian Systems. These are all free, 
globular, cup-, or platter-shaped bodies containing a central cavity, and whose calcareous wall is 
composed of elements arranged in quincunxial order. The elements lying on the outer or under 
side of the wall consist of small rhomboidal plates having four transverse rays disposed cross-wise, 
and one inwardly directed vertical ray. The systematic position of these problematical fossils 
(Receptaculites, Leptopoterion, Pasceolus, Ischadites, Polygonosphaerites) is wholly conjectural ; 
Giimbel assigns them to the calcareous algae (Dactyloporidae), and others to the Foraminifera or 
Sponges. Vide monograph by Rauf in Abhandlungen k. bayer. Akademie, IT. Cl., Bd. XVII., 1892. 
2 Milne- Edwards, H., et Haime, J., Histoire Naturelle des Corallaires; 3 vols. and atlas. 
Paris, 1857-60. 
Milne-Edwards, H., et Haime, J.. Monographie des Polypiers fossiles des terrains paléozoiques. 
Arch. du Museum, Paris. Vol. V., 1851. 
Milne- Edwards, H., et Haime, J., Monograph of the British fossil Corals. Palaeontogr. Soc., 1849-64. 
Duncan, M., ibid. 1865-69, and 1872.—Revision of the Families and Genera of the Sclerodermic 
Zoantharia or Madreporaria. Journ. Linnean Soc., London. Zoology, vol. XVIII. 1884. 
Fromentel, E. de, Introduction a étude des Polypiers fossiles. Paris, 1858-61. 
Fromentel, E. de, Zoophytes. Paléontologie francaise. Since 1861. 
Koby, F., Monographie des Polypiers Jurassiques de la Suisse. Abhandl. Schweiz. palaeont. Ges. 
1880-94, vols. VII.-XXIT. 
Koch, Ch. von, Die ungeschlechtliche Vermehrung der palaeozoischen Korallen. Palaeontographica, 
Bd. XXIX., 1883. 
Pratz, E., Ueber Septalstructur. Palaeontographica, 1882. XXIX. 
Quenstedt, F. A., Petrefactenkunde Deutschlands. Band VII., 1889. Roéhren und Sternkorallen. 
Reuss, A. H., Sitzungsberichte der Wiener Akademie, 1859, 1864, 1865, 1870 ; Denkschriften, Bd. 
VIIL., XXIII, XXVIII, XXIX., XXXI., XXXITI. 
