POSTLARVAL DEVELOPMENT. 87 



A day or two after the formation of the first cycle of entosepta, the six 

 exoccelic septa began to make their appearance, in some cases simultane- 

 ously, but in others in successive bilateral pairs from the dorsal to the 

 ventral border of the polyp (plate 2, figs. 8, 9). Fig. 8 shows that a pair 

 of septa has appeared within the dorso-lateral exocoeles, mere rudiments of 

 septa are found in the middle exocoeles, and as yet there is no indication of 

 the ventro-lateral exoccelic pair. For a long time, as shown by the coralla 

 on plates 4 and 5, the dorso-lateral pair is better developed than the middle 

 pair, and the middle than the ventro-lateral pair. The ventral pair in nearly 

 all cases remained conspicuously smaller than the other pairs. Where, how- 

 ever, the second cycle is fully developed (fig. 9), its six members are disposed 

 midway in the six interspaces between the members of the primary cycle, 

 and remain shorter than the latter. As in the entosepta the surface is spinous 

 along the edge and over both lateral faces. 



Thus, within the first week two complete cycles of septa (protosepta) 

 were developed a primary cycle, consisting of six equal entosepta, and a 

 secondary cycle of six smaller exosepta, the latter having appeared later and 

 diminishing in size from the dorsal to the ventral border. A narrow periph- 

 eral calcareous ring, unconnected with the septa, was also formed at the 

 same time. This is shown by later observations to be a marginal continuation 

 or upgrowth of the basal plate (p. 115), and therefore to be regarded as an 

 epitheca. Only the Bdwardsian mesenteries were united with the stomodaeum, 

 and of the tentacles the six exocoelic members alone were developed. 



The order of appearance of the twelve protosepta is thus in marked con- 

 trast with that of the twelve protocnemes with which they are associated. 

 The latter have been found to arise in bilateral pairs, first towards one aspect 

 of the polyp and then towards the other, and the six pairs (four macrocnemic 

 and two microcnemic) are fully established before any of the septa arise. 

 The six entosepta, on the other hand, appear simultaneously ; and such is 

 usually the case with the six exosepta in other corals, though not in 

 Siderastrea. 



The symmetrical growth of the skeletal structures, represented on plate 

 2, figs. 9 and 10, generally took place only in completely isolated polyps, 

 free to develop equally all round, and even in these irregularities were some- 

 times introduced. Among the young polyps forming the aggregated minia- 

 ture colony in fig. 5, p. 60, the septal development was scarcely alike in 

 any two. Where, as at the two extremities of the colony, one polyp partly 

 overfolds another, only half the number of septa occurs, while in the others 

 the alternation of large and small septa is inconstant ; further, the epithecal 



