76 



COELENTERATA— CNIDARIA 



PHYLÜM II 



of lime, which are developed in great quantities and remain detached in the 

 soft parts (many .Alcijonaria). In a number of forms (Corallium, Mopsea, 

 Tubipora) the spicules are firmly cemented together hy 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. 96). In some cases the sclerobase is composed entirely of 

 horny matter without admixture of calcareous secretions. In the so-called 

 "stone corals" (Fig. 97) a consistent calcareous skeleton is formed by the 

 oiiter surface of the ectoderm. At the base of the polyp between each pair 

 of mesenteries, the infolded ectoderm secretes small, round, cgi^al or irregulär 

 calcareous bodies (sderites) ; these are opposed against one another in radial 

 directions, and as others are successively laid down on top of them, upright 



Corallium rubrum Lam. (after Lacaze-Duthiers). 

 Brauch of red coral of commerce laid open along the axis, 

 and showing three polyps in section embedded in fleshy 

 coenosarc. 



Fig. 97. 



Astroides calycularis (Lamx.). Mediterran ean 

 (alter Lacaze-Duthiers). Enlarged longitudinal 

 section of polyp with calcareous skeleton. te, 

 Tentacles ; oe, Oesophagus ; me, Mesentery ; loc, 

 Mesenteric pouches ; coe, Coenosarc ; spt, Septum ; 

 col, Columella. 



partitions or septa are built up. Early in development also, after fixation of 

 the larva, the basal plate becomes calcified, owing to the secretion by the outer 

 surface of the ectoderm of numerous minute calcareous granules (calicoblasts). 

 The septa, however, grow considerably above the base, and become lodged in 

 the vertical interspaces between the mesenteries. In the same manner, within 

 the soft body-wall, a calcareous secretion may take place, binding the outer 

 borders of the septa together, and known as the wall or theca. Both septa and 

 theca are composed of minute, densely crowded calcareous bodies, in which 

 delicate calcareous fibres may be seen radiating in all directions from a central 

 dark space. And since all the calcareous bodies forming the septa have a 

 radial disposition, the calcification-centres as seen in transverse sections form 

 a dark, mostly interrupted and occasionally jagged median line, from which 

 bundles of minute fibres radiate outward in all directions. Similar calcification- 

 centres may also be found in the theca. Sometimes the median dark line is 

 uninterrupted and divides the septum into two separate lamellae. 



