ii CXIDAEIA SKELETON 99 



disc a foot-plate formed of globular calcareous grains, and thus con- 

 nects the ectoderm of the foot with the surface to which the body is 

 attached. Then from this foot-plate gradually arise, by calcareous 

 secretion from the ectoderm, radially arranged perpendicular ridges, 

 the star-ridges or sklerosepta. These are naturally covered on all 

 sides with ectoderm, and they raise the pedal disc, in as many folds 

 as there are ridges, into the gastric cavity. In the same way a cal- 

 careous tube, the theea (Mauerblatt), arises, partly by the coalescence 

 of the ends of the star-ridges, and partly perhaps also by the rising of 

 a circular wall out of the pedal disc ; this again raises the basal body 

 wall in folds into the gastric cavity, just as do the star-ridges. By 

 the formation of this calcareous tube the gastric cavity is divided 

 into a peripheral portion, lying outside the theca, and a central 

 portion, the two being in open communication above its free upper 

 edge. 



In the axis of the Coral a calcareous pillar often rises from the 

 pedal disc and projects into the gastric cavity ; this is the columella. 

 The star-ridges may fuse with this columella, they may also stand 

 out above the before-mentioned tube as ribs. By a further calcareous 

 secretion from the ectoderm round the base of the body wall, the 

 exotheea arises ; this is lined by ectoderm, and forms an outer circular 

 calcareous Avail of varying height above the pedal disc. 



The peripheral ends of the star -ridges can also unite with the 

 exotheea, though of course only by breaking through the body wall ; 

 indeed the theca may entirely or partially coalesce with the exotheea, 

 displacing the intermediate soft portions. 



Most Madreporaria, by incomplete fission or gemmation, form 

 variously -shaped Coral colonies, in each individual of which the 

 skeletal arrangement just described is repeated. Complications may 

 arise by the complete or partial fusing, or else the complete disappear- 

 ance of the thecre of the various individuals, etc. 



The star-ridges or sklerosepta never correspond in position with 

 the ordinary septa or sarcosepta, but on the contrary alternate with 

 them, so that a skleroseptum always lies between 2 sarcosepta, and 

 a sarcoseptum between 2 sklerosepta. In consequence of this the 

 sklerosepta imitate the sarcosepta in number and arrangement. 



The skeletons of the Madreporaria are either massive and close 

 (M. aporosa], or they are perforated by small cavities (M. porifera). The 

 calcareous skeletons of the fossil Paigosa had probably an origin similar 

 to that of the Hescacorallia. 



The^second kind of ectodermal Coral skeletons, the horn skeletons, 

 which are found in many Alcyonaria and in the Antipatharia, are 

 usually hollow axial skeletons ; they run through the bodies of these 

 colonial Corals., and thus take the shape of their often elegantly 

 branched stocks. It seems at first paradoxical that these axial skele- 

 tons should be ectodermal. So as to explain this fact, we shall briefly 

 describe the formation of the horn skeleton of 6-V/c/v/^/. The stocks 



