64 THE ANTHOZOA 



basal plate, and the formation of the septa. The first traces 

 of the septa are radially disposed folds of endoderm, on the 

 basal disc, one fold in each endocoele and exocoele. As twelve 

 mesenteries are present, twelve septa are formed simultaneously. 

 Beneath each fold the ectoderm becomes detached from the surface 

 of the basal plate, and is folded inwards conformably with the 

 endoderm, so that ridges composed of all three layers project 

 into the coelenteron. Between the limbs of the ectodermic folds 

 calcareous nodules are formed, and these fuse together to form 

 the septa. The septa soon fuse with the basal plate, and each 

 primary septum becomes forked at its peripheral end, so that, 

 when viewed from above, it has the shape of a Y. At a later 

 stage the septa form relatively high but thin radial plates, over 

 each of which the three layers ectoderm, mesogloea, endoderm 

 are folded. They increase in size, their peripheral ends branch, 

 and eventually the branches of adjacent septa unite with one 

 another to form a porous theca. At the same time their central 

 ends unite and form a trabecular columella. Whilst the septa are 

 being formed, and are becoming united to form a theca, a secretion 

 of carbonate of lime from the wall of the young zooid, at the 

 point where basal disc passes into body wall, gives rise to a thin 

 lamina which is continuous with the basal plate. This is the 

 epitheca ; at first it is separate from the theca, but at a subsequent 

 period is united to it by processes. Of the twelve septa first 

 formed six, viz. the exocoelic septa, grow faster than the others, 

 and thus there appear to be two cycles of alternately larger and 

 smaller septa, six in each cycle. From the foregoing account, it 

 is evident that the corallum is formed from the basal ectoderm, 

 and that it is, as it were, pushed up from below into the cavity of the 

 zooid, each part of the corallum carrying before it the three layers 

 ectoderm, mesogloea, and endoderm. Further, it is evident that 

 the theca in Astroides is not an independent structure, but is 

 formed by the coalescence of the peripheral ends of the septa. In 

 Caryophyllia, however, the theca is formed independently of the 

 septa. The development explains a feature present in many 

 Scleractiniae. The soft tissues of the zooid extend outside the 

 theca, and invest it to a greater or less extent. This extrathecal 

 extension of the soft tissues is shown in Fig. XXVIII. P. A 

 section through this region shows that the extrathecal soft tissues 

 enclose a cavity which is a part of the coelenteron, and, like the 

 latter, is divided into chambers by partitions, which are the 

 peripheral parts of the mesenteries. The extrathecal soft tissues 

 will be called the edge-zone. The extent of the edge-zone and its 

 relations to the intracalicular part of the zooid will easily be 

 understood after a study of Fig. XXVIII., and a transverse section 

 through the upper part of a zooid is shown in Fig. XXXI. 1, 



