POSTLARVAL DEVELOPMENT. 95 



already described on p. 92, the second-cycle mesenteries appearing in the 

 meantime and growing towards both the basal and the oral disc. As indi- 

 cated diagranimatically in fig. 10, d, p. 96, the new nieseuterial pairs corre- 

 spond with the space inclosed by the bifurcations of the dorso-lateral and 

 median pairs of exosepta, but seem as if about to embrace the incompletely 

 developed ventro-lateral pair. 



About this time other calcareous upgrowths began to appear peripherally, 

 midway within the exocoelic bifurcations, and necessarily inclosed within 

 the eutocceles of the second-cycle mesenteries. Plate 3, fig. 15, gives the 

 discal view of such a living polyp in which the six pairs of second-cycle 

 mesenteries had been developed for some time. The new mesenteries, still 

 varying in size in agreement with their order of appearance, have now 

 begun to extend along the periphery of the disc, and the latter is resting 

 upon the upper edges of the septa with the tentacles fully expanded. The 

 septa are clearly seen through the trausparent disc. Within the bifurcation 

 of each dorso-lateral and middle exoseptum has appeared an additional free 

 septum, included within the entoccele of the second-cycle mesenteries, and 

 in the same radius as the original exoseptum. Moreover, the exoccelic 

 bifurcations are now seen to be wholly exocoelic in position, situated in the 

 chambers between the pairs of the first and the second-cycle mesenteries ; 

 in the dorso-lateral sextants the forkings are free from the original median 

 exoseptum, but in the middle sextants they are united. The primary ento- 

 septa remain simple straight bars, and such is yet the condition of the 

 ventro-lateral exosepta. Fig. 10, e, p. 96, is a diagrammatic representation 

 of the polyp and corallum at this stage. 



The new formations within the second-cycle entocceles suggest an inde- 

 pendent series of septa, and subsequent stages show that they represent the 

 second-cycle entosepta. The polyp of plate 3, fig. 15, in fact, displays the 

 early stages in the development of the second C3 r cle of entosepta and the 

 establishment of a third cycle of exosepta, and in such a manner that the 

 actual relationships between the new septa and the second-cycle mesenteries 

 admit of no misinterpretation. The entosepta of the second cycle appear 

 peripherally as separate formations, but in their later growth centrally, as 

 shown on plate 5, figs. 28, 29, and fig. 10,7^ p. 96, they come into union with 

 the primary exosepta, and the two then appear as a single continuous structure. 

 Further, each bifurcation of a primary exoseptum forms a new exoseptum, 

 belonging to a third cycle, and may be either distinct or united by its inturned 

 edge with a second-cycle eutoseptum. 



