9 o 



J. E. DUERDEN. 



than the mesenteries, which in their turn have also increased in 

 size. Both limbs of the bifurcated septum are now free from the 

 middle portion ; they represent two independent exosepta becom- 

 ing distinct from the single exoseptum of a previous cycle (cf. 

 Fig. 6) of which they were originally continuations. 



Fig. c is from a section through the column wall, the septa at 

 this level being exsert, that is, extending above the calicinal wall. 



e f Q 



FIG. 7 ( f t-g)- Series of diagrammatic figures illustrating the developmental 

 relationships of a pair of third cycle mesenteries and a third cycle entoseptum, in 

 association with a bifurcated third cycle exoseptum. 



The relationships shown are the same as in the previous figure, 

 but the inner radial portion of the original third cycle exoseptum 

 has almost disappeared. Fig. f is through the column wall 

 (lower) and disc (upper). The mesenteries now extend from one 

 wall to the other, the polyp being in the retracted condition, and 

 the entoseptum is still smaller than the exosepta, one on each 

 side of it. It is manifest that in the later growth the entoseptum 

 will extend more centrally, and come into union with the original 

 third cycle exoseptum which is in the same radius (Fig. g), 

 exactly as in the larval polyp shown in Figs. 5 and 6. 



