250 CCELENTERATA. 



ber, and divide it into a series of storeys placed one above the other, 

 the only living portion of the coral being above the last-formed tabulae. 

 In some cases, the tabulae are "incomplete," and are merely flat 

 tongue-shaped plates which extend from the inner surface of the 

 theca transversely into the visceral chamber. In other cases (as 

 in Mickelinia), the tabulae are imperfect, and become united with 

 one another so as to form a sort of vesicular tissue. In still other 

 cases (as in Syringopord) the tabulae are funnel-shaped, and fit into 

 one another from above downwards. 



As regards the process of development, the ordinary sclerodermic 

 corallum is the result of the secretion of carbonate of lime by a special 

 layer of ectodermal cells (" calicoblast layer," fig. 130,^), and is therefore 

 truly external to the body of the polype by which it is produced. If the 

 formation of the corallum be observed in the embryo of Astroides, it is 

 found that the first step is the secretion of a ring-shaped " basal plate " 

 (fig. 130, bd) between the lower surface of the polype and the substance to 



Fig. 130. — Diagrammatic vertical section ox a young polype or Astroides calicularis, showing 

 the formation of the corallum, greatly enlarged, ba, Basal plate, produced by a special layer 

 of ectodermal cells (ca) ; ep, Lateral wall of the corallum ; se, One of the septa, pushing before 

 it a fold of the entire body-wall ; ec, Ectoderm ; en, Endoderm ; me, Mesoderm (" supporting 

 lamella ") ; mn, Mesentery ; ce, Gullet. (Slightly altered from Bourne, and based on a figure 

 given by von Koch.) 



which it has attached itself. The formation of the septa is subsequent to 

 that of the basal plate, these structures appearing as radially-disposed 

 ridges or folds of the base of the polype, each fold (fig. 130, se) being com- 

 posed of the three constituent layers of the body-wall. The septa are 

 thus, like the basal plate, really external to the polype ; but in process 

 of growth they carry up with them the folds of the body-wall by which 

 they are covered, and they ultimately come to project into the inter- 

 mesenteric chambers, and to have all the appearance of being internal 

 structures. 



The theca is not an independent structure, as is the basal plate, but 

 seems to be usually the result of the bifurcation of the peripheral ends 

 of the septa, and the lateral fusion of these (fig. 127, a, /). In some cases 

 the external investment of the corallum seems to consist solely of the 

 thickened outer ends of the septa (as in Streptelasma, fig. 127, b) ; but in 



