POSTLARVAL DEVELOPMENT. 



103 



The first two figures (0, b) correspond with plate 9, figs. 54~5 6 > of tne 

 sections. The exoseptum is here simple, but bifurcated towards its periph- 

 eral extremity. At first no mesenteries are included within the angle of 

 bifurcation, but a pair has appeared in fig. 12, b. In relation to the mesen- 

 teries the two limbs are clearly exocoelic, and may even at this stage be 

 regarded as two individual exosepta. 



In fig. 12, , one exoseptum has become distinct, and a short septum has 

 appeared midway between the two older septa (cf. plate 9, fig. 57). Clearly 

 this is a new entosepturn arising a little later than the new pair of mesen- 

 teries. In fig. 12, d, the right exoseptum has also separated from the more 

 central portion which is along the same radius as the entoseptum (cf. plate 

 9, fig. 58). The next figure, *, shows the exsert septa (cf. plate 9, fig. 59); 

 the central part is becoming smaller, while it has disappeared in the last 



FIG. ia. Series of diagrammatic figures illustrating figs. 54-60 on plate 9. 



figure, f (cf. plate 9, fig. 60). The entoseptum throughout extends the 

 shortest vertical distance of the three, and so far as the development has pro- 

 ceeded remains the smallest radially. Clearly when growth is completed 

 the three septa will be related as shown in the six groups of fused septa in 

 fig. io,/, p. 96, which is the usual relationship of the exosepta and entoseptum ; 

 the entoseptum in its further growth will have extended more centrally and 

 fused with the central portion of the original exoseptum, and the two exosepta 

 will turn inwardly to unite with it. 



The series of sections plainly demonstrates that a new entoseptum 

 arises shortly after a new pair of mesenteries, not in advance of it, and the 

 two peripheral limbs of a bifurcated exoseptum become distinct, so that each 

 constitutes a new exoseptum. The new entoseptum takes the place of the 

 inner simple part of an exoseptum situated in the same radius. 



The members of the third-cycle entosepta of the older polyp are new 

 formations which take the place of the earlier third-cycle exosepta. The 

 dorsal and ventral moieties of the bifurcated periphery of the third-cycle exo- 



