POSTLARVAL DEVELOPMENT. 



105 



The order of appearance of the twelve pairs of third-cycle mesenteries 

 has been fully described on p. 83. It is found that the members arise after 

 the formation of the six second-cycle pairs as unilateral pairs in a twofold 

 succession. Normally a pair appears within the dorsal exocoele of each 

 sextant, successively from the dorsal to the ventral border of the polyp ; then 

 another pair appears within the ventral exocoele of each sextant, following 

 the same succession as the first series. 



Ilia 



Ha 



nid 



[lid 



Illc 



Illc 



FIG. 13. Diagram illustrating ihe order of development of the first three cycles of septa (I-III). 

 The small letters accompanying the Roman numerals indicate the sequence of the indi- 

 vidual septa in the cycle. 



The normal sequence of the third-cycle mesenteries in Siderastrea 

 being established, we are justified in assuming that a like succession will be 

 maintained by the third-cycle entosepta, seeing that they arise shortly after 

 the mesenteries with which they are associated. This is shown in fig. 13, 

 p. 105, by the numerals iiia-mf. The figure also represents the normal 

 sequence for all the entosepta of the first three cycles. The six primary 

 septa appear together, a cycle at a time ; the six members of the second order 

 arise bilaterally in a simple dorso-ventral succession ; the twelve members of 

 the third also arise bilaterally in a dorso-ventral succession, but in two 

 series first, a series of six within the dorsal of the two interspaces in each 



