98 SIDERASTREA RADIANS. 



corallum there are indications still remaining of the distinctness of the 

 peripheral and central parts of the entosepta, as in the ventral sextants of 

 fig. 28, plate 5, where the entoccelic moiety is still free from the exocoelic 

 portion. The fusion of the two parts is a mechanical necessity in the 

 process of growth, seeing that both are on the same radii. It is manifest 

 that it is only by securing such intermediate stages that the true character 

 of the septa at the mature stage can be understood. When the secondary 

 entosepta have come into fusion with the primary exosepta there remains 

 no means of distinguishing their compound origin, and the ontogenetic and 

 phylogenetic significance of the secondary entosepta is obscured. 



The twelve exosepta now forming the tertiary cycle undoubtedly arise as 

 continuations of the bifurcations of the six primary exosepta, but in 

 many places, as on plate 5, figs. 27 and 28, they still show a considerable 

 amount of distinctness. Regarding them as continuations of the two forks 

 of each primary exoseptum, it is manifest that they are to be considered 

 as appearing in advance of the entosepta of the second cycle, a relationship 

 which need not be wondered at, considering that in this species the exo- 

 teutacles are also found to arise in advance of the entotentacles. Originally 

 forming the second cycle, the exosepta now constitute the third septal cycle, 

 their place in the sequence being taken by the new second cycle of entosepta. 



It is thus manifest that in the course of development of a coral the 

 exosepta of a former stage do not become the eutosepta of a later stage ; 

 the latter are new formations appearing after the establishment of the 

 mesenteries with which the}' correspond, and consequently the mesenteries 

 and their included entosepta have the same ordinal value. The exosepta, 

 on the other hand, have no ordinal value ; they appear at each cyclic stage, 

 always constituting the outermost cycle. They are, in a measure, temporary 

 structures, predecessors of the entosepta, until the limit of growth of the 

 polyp is reached ; they serve as integral parts of the septal system of the 

 coral during all its intermediate stages, and are then overgrown by later 

 permanent septa.* 



In some corals the exoseptal predecessors appear to continue their independent growth in siltt 

 without losing their individuality as skeletal structures in the central extension of the entosepta. In 

 these cases the entosepta do not grow far enough centrally to fuse completely with the exosepta already 

 there. I believe it will be found that this is the true nature of fall which are found in some corals as 

 small septum-like plates in front of the larger septa. The fact that pali seem not to occur before the 

 primary cycle of six septa, but only before those of later origin, is what we should expect if this surmise 

 be correct. The primary entosepta have never had exocoelic predecessors, as is the case with the later 

 entosepta. The pali would thus represent the persistent exocoelic predecessors of the entosepta beyond 

 the primary cycle. 



