68 



COELEXTERATA CNIDARIA 



SUB-BRANCH II 



conducting into the gastric cavity. The outer covering of the body, whose 

 parts are designated as wall, oral disk, and pedal disk, are constituted of 

 ectoderm and cntoderm, between which is a thin layer of mesoderm (Mesogloea). 

 Six eiaterejnore radially disposed vertical partjt/ions (wseT^mesX-lTigs. 98, 99), 



_ 



projecting inwardly from the body-wall, divide the gastric cavity into a serie's 

 of radiating compartments (mesenteric pouches}. The mesenteries are prolonged 

 upwardly as the hollow, muscular tentacles; while the generative organs are 

 attached to their faces near the lower end of the body. The mesenteries are 

 mvrivd on both sides with muscular tissues, and bear mesenteric filaments on 

 their curled inner edges. On one side of the mesenteries the muscle fibres are 

 transversely directed, on the other longitudinally. The longitudinal system 

 is usually considerably folded and thickened ; and the disposition of these 

 muscular portions (Miiskelfahife) is of great importance from a systematic stand- 



point, since it reveals 

 the bilateral symmetry. 

 of many Anthozoans, 

 and enables one readily 

 to identify the anti- 

 meres. If a polyp in- 

 dividual be cut in two 

 by a plane passing 

 through the longer axis 

 of the mouth -opening, 

 then, in the Octocorll<i 

 (Fig. 98), the mesen- 



teries of the right half 

 W JH h ave a ]} the milS- 



FIG. OS. 



Diagrammatic section of 

 the soft parts of an Octo- 



i-nriil/ii (Ali'ifnii in Hi), x, Oeso- 

 phagus ; 1, 2, 3, 4, Mesen- 

 teries of the left side (after 

 R. Hertwig). 



Diagrammatic section of the soft parts of 

 a Hexacomlla. In the upper half (above the 



line a 5) the section passes through oeso 

 phagus s ; in lower half, beneath 1 " 

 Corallum indicated by heavy lines. 



beneath the same. Clllar thickenings dis- 

 ivy lines, r, direc- -i ,1 . > , -i -i 



tive mesenteries. posed On the Hght-hai id 



side, and those of the 



left on the left-hand side. In the Hexacomlla (Fig. 99) the mesenteries are 

 grouped in pairs, with the muscular thickenings of any pair facing each other. 

 T\v pairs, however (those corresponding with the opposite extremities of the 

 longitudinal mouth), form often an exception to this rule, since these have the 

 muscular thickenings placed on opposite sides. These are called the directive 

 mesenteries, and serve to indicate the longitudinal axis of the body. 



Only a few Anthozoa have permanently soft bodies ; the majority secreting 

 calcareous, horny, or partly horny and partly calcareous structures, termed the 

 skeleton or corallum. The simplest form of corallum is that composed of 

 microscopic, round, cylindrical, acerate, or tuberculated spicules of carbonate of 

 lime, which are developed in great quantities in the mesoderm and remain 

 detached in the soft parts (many Alcymaria). In a number of forms (Cornllhoti, 

 Mopseii, TiibijHU'a) the spicules are firmly cemented together by means of a 

 calcareous or horny connective substance, in such a manner as to form tubes 

 ( Tubipora), or, when the secretion takes place chiefly at the base, a sclerobase, 

 or axis. Surrounding the axis is the soft coenosarc in which the polyps of the 

 colony are embedded (Fig. WQ 1 ). In some cases the sclerobase is composed 

 entirely of horny matter without admixture of calcareous secretions. In the 

 so-called "stone corals" (Fig. 101) a consistent calcareous skeleton is formed 

 by the outer surface of the ectoderm. At the base of the polyp between each 



