﻿I IO SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 47 



The skeleton never appears until after fixation of the larva. It 

 makes its first appearance in the form of minute plates or granules, as 

 an ectoplastic product of the ectodermal cells (calicoblasts) of the 

 base. A flat, circular, basal plate is formed by the union of these, 

 and may be produced upward at the edge as the epitheca, while from 

 its inner or polypal surface the septa begin to appear as vertical 

 upgrowths formed within invaginations of the basal disk of the 

 polyp. The skeletal cup first formed is known as the prototheca. 



Like the tentacles, the first two cycles of septa (protosepta) may 

 appear simultaneously, or the cycle of six entosepta may arise in 

 advance of the cycle of six exosepta. The order of appearance of the 

 later cycles is not yet thoroughly understood, the relative sizes in the 

 mature corallum by no means indicating the actual order of develop- 

 ment. As in the case of the mesenteries, the radial plan of the mature 

 or less definite dorso-ventral or antero-posterior succession. Further- 

 more, as in the case of the tentacles, the exosepta remain exosepta 

 throughout the course of their development, always constituting the 

 outermost cycle. The entosepta beyond the primary six follow the 

 same succession of growth as the mesenteries, so that the order 

 assigned the secondary and tertiary mesenteries in figure 13 will also 

 hold for the septa. 



EXTINCT CORALS 



Studies on the septal development of the extinct Paleozoic corals, 

 known as the Rugosa or Tetracoralla, reveal that in these early 

 forms the primary septal plan was hexameral, like that of modern 

 forms, but the later septal development is altogether different from 

 anything found in recent corals. The metasepta appear in successive 

 bilateral pairs, within only four of the six primary interseptal cham- 

 bers, and the corallite retains a bilateral symmetry throughout its 

 developmental stages, though afterward it may attain radial sym- 

 metry. Only two cycles of septa are ever present, a larger and a 

 smaller (dicyclic), though some members of the primary cycle may 

 differ in size from the remainder. The larger septa are entosepta and 

 the smaller exosepta ; only the former have any definite ordinal sig- 

 nificance, the exosepta appearing at different times in different 

 species. 



On account of the manner of appearance and arrangement of the 

 septa the Rugosa must be clearly separated from modern hexamerous, 

 polycyclic Madreporaria ; of all the living zoantharians they are most 

 septa is derived from structures which appear bilaterally, in a more 

 closely related to the zoanthids among- the actiniarians. 



