POSTLARVAL DEVELOPMENT. 97 



secondary entosepta are new formations, arising within the second-cycle 

 entocoeles independently of other septa ; the second-cycle mesenteries never 

 embrace the exocoelic protosepta as such, but only after their fusion with 

 the new entosepta. (3) The appearance of the second-cycle entosepta is 

 in a dorso-ventral manner, not simultaneous, a cycle at a time. 



Other coralla exhibit different stages, all pointing to the same conclu- 

 sion. That shown diagrammatically in fig. 9, A, p. 89, is in somewhat the 

 same stage as fig. 10, e. In the upper right sextant the second-cycle 

 entoseptum (u) is yet very distinct from the exocoelic bifurcations, but an 

 irregularity in the rate of growth is exhibited by the corresponding left 

 sextant, in that no entoseptum has yet formed. The growth in the two 

 middle sextants is in advance of it ; each of the sextants contains a primary 

 exoseptum, a secondary entoseptum, and two third-cycle exosepta as distinct 

 formations. As before, the development within the two ventro-lateral sex- 

 tants lags behind that of the middle and dorso-lateral sextants. Fig. 9, jr, 

 from another corallum, also shows important intermediate stages. 



The next stages available are the coralla of the oldest polyps reared, two 

 of which are represented on plate 5, figs. 28, 29, and diagrammatically in fig. 

 9, j\ p. 89, and fig. 10, /, p. 96. Each corallite now consists of three com- 

 plete orders or cycles of septa, the development within the two ventral 

 sextants having reached the same stage as that within the dorsal and middle 

 sextants. The first order contains six large septa radially disposed, the 

 directive septa a little larger than the others. The second order also 

 includes six septa, turned inwardly in such a manner that the two dorsal 

 fuse with the dorsal directive septum, the two middle fuse with the dorso- 

 laterals of the first order, and the two ventral with the ventro-laterals of the 

 first order. The third cycle contains twelve septa, the two adjacent members 

 fusing with each of the six septa of the second order. The coalescence of 

 the septa in this way imparts a bilateral symmetry to the calice which other, 

 wise would be radial. Further, adjacent septa are now for the first time 

 joined by synapticula. The polyps (plate 3, fig. 17) forming the coralla bore 

 two cycles of mesenteries, so that the first and second orders of septa are 

 entosepta, while the members of the outermost cycle are exosepta. 



Previous stages have demonstrated that the members of the second 

 order of septa, although now continuous bars, have really a twofold origin. 

 The peripheral part of each appeared as a separate septum within an entoccele 

 of the second-cycle mesenteries and, later continuing its growth centrally, 

 fused with an exoseptum which was originally a constituent of the second 

 cycle of septa, so that now the two appear as a single septum. In each 



