POSTLARVAL DEVELOPMENT. 91 



weeks old, in which only six pairs of primary mesenteries and two cycles of 

 tentacles were present, while fig. 9, ^, p. 88, is its diagrammatic representa- 

 tion. Each septum now bears a closer resemblance to the septa in the 

 mature corallite, being narrow centrally and broad peripherally, with spiuous 

 projections over the entire surface. The general relationships of the septa 

 to the mesenterial pairs could be made out before maceration ; hence there is 

 no uncertainty as to the orientation of the corallum. The dorsal and ventral 

 directive entosepta present a somewhat bifurcated peripheral extremity, the 

 two limbs being nearly parallel, but the four lateral entosepta are simple 

 bars, with thickened peripheral ends formed from the enlargement and 

 complete fusion of the originally separate elements. On the other hand, 

 the dorso-lateral and middle pairs of exosepta are strongly bifurcate.! periph- 

 erally, while the members of the ventral pair retain their simple character, and 

 are by far the smallest of the six pairs. The peripheral limbs in the bifurcated 

 exosepta may be quite separate from the single radial piece, though usually 

 they are joined. It is readily seen how by the enlargement and fusion of the 

 many detached fragments within each mesenterial space of plate 2, fig. 12, 

 such a septal condition as that of plate 4, fig. 23, has been obtained. 



With the increase in thickness of the septa the interseptal loculi are 

 correspondingly diminished, but no actual syuapticular unions are yet formed 

 across the interspaces. The dorso-lateral exosepta bend laterally and nearly 

 fuse centrally with the dorsal directive septum ; the middle exoseptum on 

 each side approaches the corresponding dorso-lateral entoseptum and fuses 

 with it, while the small veutro-lateral exoseptum on each side is fused with 

 the ventro-lateral entoseptum (fig. 9, ^, p. 88). 



Much older corallites, but still at nearly the same stage of development, 

 are shown on plate 4 (fig. 24) and on plate 5 (figs. 25-27), and diagrammati- 

 cally by fig. g,/, p. 89. The dorso-lateral exosepta are now fused with the 

 dorsal directive septum, the middle exosepta with two dorso-lateral entosepta, 

 and the ventro-lateral exosepta with the ventro-lateral entosepta. The exo- 

 septa, generally, are not so strongly bifurcated as in the corallite on plate 4, 

 fig. 23 (fig. 9, <?, p. 88). Fig. 10, -</, on p. 96, shows the diagrammatic rela- 

 tionships of the septa to the mesenteries throughout these early stages. 



The general impression produced by the corallum at this stage, as seen 

 under a simple lens or low power of the microscope, is that of two alternating 

 cycles of septa a larger and a smaller. The tendency to fusion of adjacent 

 septa, exosepta with eutosepta, which was found to be so marked a feature 

 of the adult corallite, is already exhibited by the two cycles, and gives a 

 bilateral symmetry to the calice. 



