16 Coelenterata. 



12 pairs, 6 pairs arising in succession from one aspect of the polyp to the other 

 and all on the dorsal aspect of the first cycle ; the remaining 6 pairs appear in the 

 same succession but on the ventral aspect. Polyps arising by discal fission have 

 no directive mesenteries and lose the cyclical regularity characteristic of the sexu- 

 ally reproduced polyps and of the polyps which originate by columnar gemmation, 

 in which directive mesenteries always occur. In P. the additions of mesenteries 

 beyond the 6 primary pairs are always made at either the ventral or dorsal regions 

 of the polyp and within an axial exocoel. The new mesenteries arise in bilateral 

 complete pairs, the latest-formed appearing within the entocoel of the previously 

 formed pair. The manner of appearance of the secondary mesenteries in P. differs 

 from that in all other known coral polyps. In the former the additions take place 

 in bilateral pairs at only one region (dorsal or ventral) and within eutoccels, whereas 

 in other coral polyps the additions are made all round the polyps in unilateral 

 pairs and within exocoelic chambers. The 3 principal divisions of the Actiniaria 

 are characterised by a difference in the origin of the secondary mesenteries. In 

 Hexactiniae they appear in unilateral pairs within the exocoels of the primary cycle ; 

 in Zoantheae they arise only in two regions, one on each side of the ventral direc- 

 tives; in Ceriantheae the mesenteries beyond the 4 Edwardsian pairs arise in bi- 

 lateral pairs in the dorsal region of the polyp each within the entoco?! of the pre- 

 viously formed pair. The Madreporaria resemble the Hexactiniae in the arrange- 

 ment of their mesenteries, while of all Actiniaria the Ceriantheae most nearly 

 approach the condition seen in P. The Ceriantheae bear much the same relation to 

 the Hexactiniae which P. does to the rest of the Madreporaria. - - In young polyps 

 of Man. areolata the first 6 septa appear simultaneously within the entocoels of 

 the larva at the stage when it has 12 mesenteries. At no stage do exocoelic septa 

 appear in this coral. In polyps arising by gemmation, as in Ma., the 6 pairs of 

 primary mesenteries are established before the corresponding septa are formed. The 

 6 primary septa appear to arise simultaneously. There may be an interval of se- 

 veral weeks in the life of the young polyp between the completion of the primary 

 septa and the appearance of the secondary mesenteries and septa. The appearance 

 of the secondary septa has up to the present been traced only in buds, but there 

 seems no reason for supposing that the order is different from that in sexually pro- 

 duced polyps. These septa appear in pairs practically simultaneously within each 

 pair of secondary mesenteries and they thus differ from the primary septa which 

 are formed a cycle at a time. The actual order of succession already determined 

 for the secondary mesenteries is followed by the corresponding septa. In nearly 

 all corals with several hexamerous cycles of secondary septa the first cycle of 6 

 occurs within the entocoels of the 6 primary mesenteries, the second cycle within 

 the entoccels of the 6 pairs of first cycle of metacnemes, the 12 septa of the third 

 cycle within the 12 pairs of second cycle of metacnemes and so on, but the septa 

 of the last cycle are exocoslic. In the fissiparous genera Man., Meandrina, Iso- 

 phyllia and Favia only entoccelic septa are present. The mode of appearance and 

 relationship of the secondary septa corresponds with the relationship established 

 by Lacaze-Duthiers and Faurot for the tentacles of Actiniaria, which the Author 

 finds to be the same for the tentacles of the Madreporaria. In P. the secondary 

 septa follow the secondary mesenteries in close succession, an entocoelic and an 

 exocoelic representative on each side. There are only two orders of septa - - ento- 

 coelic and exocoelic, a multicyclic condition such as is characteristic of most other 

 recent corals being impossible. The bilateral symmetry of the septa, the alterna- 

 tion of only large and small septa and their growth from one or more special re- 

 gions are all characters of primary importance among many of the Palaeozoic co- 

 rals and sometimes with a tetrameral arrangement serve to sharply distinguish these 



