. The annals and magazine of natural history : zoology, botany, and geology. mental pairs are complete, the fifth and sixthpairs remain permanently free from the stomodreum. Wherein other genera additional mesenteries occur the next sixnairs are arranged in a cycle alternating with the primary Morphology of the Madreporaria. 383 six; in some cases, as in Phyllangia and some speciesof Orbicella, the second order mesenteries become unitedwith the stomodgeum, but usually they are free. Where athird order of mesenteries is fully developed it consists oftwelve pairs, alternating with the twelve pai
. The annals and magazine of natural history : zoology, botany, and geology. mental pairs are complete, the fifth and sixthpairs remain permanently free from the stomodreum. Wherein other genera additional mesenteries occur the next sixnairs are arranged in a cycle alternating with the primary Morphology of the Madreporaria. 383 six; in some cases, as in Phyllangia and some speciesof Orbicella, the second order mesenteries become unitedwith the stomodgeum, but usually they are free. Where athird order of mesenteries is fully developed it consists oftwelve pairs, alternating with the twelve pairs which consti-tute the first and second orders, and by a difference in size isrecognizable as a distinct cycle. A fourth order of mesen-teries would consist of twenty-four pairs, alternating with allthe previous pairs, and a little shorter in radial extent, thusconstituting a separate cycle. The typical condition of a coral polyp wuth four completehexameral cycles of mesenteries is diagrammatically repre-sented in fig. 1, but such perfect regularity as is here indicated. Diagrammatic mesenterial arrangement in a simple polyp of Mauicinaareolata in wbich four orders of mesenteries (I-IV) are present,arranged in three cycles. D, D, directives. Septal invaginationsomitted. The section is placed with the dorso-ventral axis lateralinstead 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 regularlycyclical and hexameral. The septa in all the representatives of the above genera arearranged in hexameral cycles in close conformity with the 384 Dr. J. E. Duerden on the mesenteries, a single septum within each entocoele and eachexocoele. An order (primary, secondary, tertiary, &c.) ofentosepta corresponds with each order of mesenteries, wh
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