86 



J. E. DUERDEN. 



small independent septa ; they suggest a new series of entoccelic 

 septa, appearing in a dorso-ventral sequence like the mesenteries 

 with which they are associated. 



Later these new septa extended more centrally, and necessarily 

 came into union with the simple inner portions of the septa which 

 originally constituted the exoccelic second cycle. Several of the 

 polyps were reared until the new second and third orders of 

 septa were fully established, when they presented the arrangement 

 shown in Fig. 6. The peripheral, primarily independent septa 



FIG. 6. Completion of the first three cycles of septa. 



within the second cycle entocceles have all become continuous 

 with the median part of the original second cycle exosepta, and 

 along with them now constitute the permanent second cycle of 

 entosepta ; while the bifurcations of the exosepta now consti- 

 tute the third cycle of twelve septa and are seen to be exosepta. 1 

 It will also be seen that the growth within the ventral system 

 has now attained the same stage as that within the middle and 

 dorsal systems, so that the corallite as a whole presents nearly 

 perfect radial symmetry. 



The stages thus passed through are of great importance in 

 their bearing upon several obscure points in coral development 

 and morphology, and call for fuller consideration. In the first 



1 In the adult corallite of Siderastrea radians the exosepta are fused by their inner 

 end-; with the entosepta as represented in Fig. 6 and Fig. ~]g. 



