Morphology of the Madreporaria. 



383 



six ; in some cases, as in Phyllangia and some species 

 of Orbicella, the second order mesenteries become united 

 with the stomodgeum, but usually they are free. Where a 

 third order of mesenteries is fully developed it consists of 

 twelve pairs, alternating with the twelve pairs which consti- 

 tute the first and second orders, and by a difference in size is 

 recognizable as a distinct cycle. A fourth order of mesen- 

 teries would consist of twenty-four pairs, alternating with all 

 the previous pairs, and a little shorter in radial extent, thus 

 constituting a separate cycle. 



The typical condition of a coral polyp wuth four complete 

 hexameral cycles of mesenteries is diagram matically repre- 

 sented in fig. 1, but such perfect regularity as is here indicated 



Diagrammatic mesenterial arrangement in a simple polyp of Mauicina 

 areolata in wbich four orders of mesenteries (I-IV) are present, 

 arranged in three cycles. D, D, directives. Septal invaginations 

 omitted. The section is placed with the dorso-ventral axis lateral 

 instead of vertical, in order to facilitate comparison with figs. 2 and 3. 



is rarely exhibited by the later cycles. Further, many polyps, 

 even when mature, do not possess the entire number of mesen- 

 teries necessary to complete the last cycle which is begun; 

 yet in every case the sequence, so far as it goes, is regularly 

 cyclical and hexameral. 



The septa in all the representatives of the above genera are 

 arranged in hexameral cycles in close conformity with the 



