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. 



Fig. 13. Diagram illustrating ihe order of dovelupmmt of the tirit 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 i\ia-\i\f. The figure also represents the normal 

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

 septa appear together, a C3^cle 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 



