MORPHOLOGY OF THE MADREPORARIA. 



took place by the direct extension of the septa already developed, 

 and in others by the deposition of separate skeletal nodules. The 

 new growth was arranged in a V-shaped manner, the angle of 

 the V being larger in the exosepta than in the entosepta (Fig. 3). 

 In the diagrammatic figure the additions are all represented as 

 separate calcareous fragments, but no constancy was apparent in 

 the different septa as to the freedom or fusion of the extensions 

 at this stage. In all the polyps the enlargement of the ventral 

 exosepta was much behind that of the dorsal and middle exosepta, 

 and usually it could be seen that the middle exosepta did not 

 enlarge as rapidly as the dorsal. 



At a somewhat later stage, each septum became a continuous 

 structure, owing to the complete fusion of the free nodules in the 



FIG. 3. Stage showing the peripheral 

 enlargement of the septa which occurs in 

 a bifurcating manner by the addition of 

 separate nodules. The ventral exosepta 

 grow more slowly than the middle and 

 dorsal pairs. 



FIG. 4. Further enlargement of the 

 septa by fusion of the nodules, and estab- 

 lishment of the six pairs of second cycle 

 mesenteries ; the latter, like the exosepta, 

 decrease in size from the dorsal to the 

 ventral border. 



process of growth. The entosepta then appeared as simple 

 formations, much broader peripherally than centrally, though the 

 two directive entosepta showed traces of the earlier bifurcation 

 longer than the four lateral entosepta ; the exosepta, on the other 

 hand, remained strongly bifurcated, with the exception of the 

 ventral pair, the members of which enlarged comparatively little 



(Fig- 4). 



A similar bifurcated stage, due to the appearance of indepen- 

 dent calcareous nodules, occurs in the development of Astroidcs, 



