MORPHOLOGY OF THE MADKEPOKARIA. QI 



The series of sections thus reveals that the third and fourth cycles 

 of septa and the third cycle of mesenteries are related in the same 

 manner as are the second and third cycles of septa and the second 

 cycle of mesenteries ; the third cycle entosepta have third cycle 

 exoccelic antecedents. The results may be arranged as follows : 



(a) The originally simple third cycle exosepta, themselves 

 formed as bifurcations of simple second cycle exosepta, become 

 bifurcated at their peripheral extremity. 



(//) Within each bifurcation there appears a new pair of third 

 cycle mesenteries, and then within the entocosle of the pair is 

 formed a third cycle entoseptum. 



(<:*) The new entoseptum fuses with the central portion of the 

 third cycle exoseptum, while the bifurcations of the latter con- 

 stitute two new exosepta of the fourth cycle, and are fused at 

 their inner extremity with the entoseptum embraced by them. 



Presumably the same process as above outlined will be followed 

 within the two exoccelic chambers of each sextant of the polyp, 

 so that in the end there will be twelve entosepta forming the 

 completed new third cycle and twenty-four exosepta forming the 

 completed new fourth cycle. 



The actual sequence according to which the third and fourth 

 cycles of septa are formed remains to be noticed. 



The septa alone in the dried corallum are insufficient for this 

 purpose, as they afford no certain means by which the directive 

 axis can be determined, and from this the dorsal and ventral 

 borders of the calice. It has been shown, however, that the 

 mesenteries associated with the entosepta appear in pairs only a 

 little in advance of the entosepta within them ; therefore if the 

 sequence of the mesenteries be determined it can be assumed 

 that the septa follow the same order. 



From colonies of Sidcrastrea sections of a large number of 

 bud polyps at different stages of development have been pre- 

 pared, and from these it has been possible to determine the order 

 of appearance of the twelve pairs of third cycle mesenteries. 

 This is indicated in the series of diagrammatic figures in Fig. 8 

 (ad\ In Fig. 8 a, in addition to the primary and secondary 

 cycles, a pair of third cycle mesenteries (III.) has appeared on 

 each side of the median axis, situated in the exoccele between 



