MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 515 



The plane of fission coincides with the primaiy directive plane which passes through the 

 primary dorsal and ventral directive entocceles and longer oral axis, and divides the enlarged 

 polyp into equal halves with twelve mesenteries to each; of these, six are the protocnemes and 

 six are new formations. 



As the order of appearance of the mesenteries beyond the protocncmic stage in Porites 

 differs from that in other coral polyps, so its method of fission is altogether different. In most 

 corals the plane of fission is at right angles to the directive or median axis of the polyp, whereas 

 in Porites it is along the directive plane; each of the two primar}^ daughter polyps in ordinary 

 fission has only one pair of directives, but in Porites the mesenteries are arranged exactly as 

 in the larval polyp, and each fission polyp bears two pairs of directives. (See foot-note, p. 496.) 



FISSION IN MADEEPORA. 



When describing, in a recent paper", the method of addition of new mesentei'ies in 

 Madrejxira, beyond the protocnemic stage, the process of fission was also noticed, so that for 

 the present purpose it is only necessary to bi'iefly reiterate the facts there brought forward. On 

 examining most colonies of Madrepora with a lens, a few polyps are found which are slightly 

 larger than the others, and bear more than the usual twelve tentacles, any even number from 

 sixteen to twenty-four being represented. The tentacles form, however, only a single cycle, as 

 in ordinary polj'ps (PI. I, fig. 1 h). Such polyps seem to occur anywhere over the surface of 

 a colony, wherever vigorous growth is in progress, but have not lieen found in the oldest parts of 

 colonies, nor on the under surface of palmate colonies, where conditions of growth are not 

 very favorable. 



Fig. 18a. 

 Madrepora, — Figs. 18. Series of diagrammatic figures illustrating polypal fission and the manner of increase of the mesenteries beyond 

 the protoenemic stage. Two stomoda-al tubes are present from the beginning, either connected by one or two mesenterial strands 

 or altogether distinct, a, Four new bilateral pairs (A-D) are present within the ventral directive entoccele. and two others connect 

 the two stomodseal tubes. 



Most of the larger polyps are strongly oval, the longer axis being at right angles to the 

 axial-abaxial plane. Out of forty or fifty enlarged polyps examined, only one or two did not 

 already display two oral apertures, and of nearly thirty specimens scctionized transversel}' each 

 bore two distinct stomodieal tubes. Compared with the enlarged polyps of Portfes those of 

 Madrepora are therefore characterized by the earh' production of the bioral condition. 



Transverse sections reveal that in practically every case twenty-four mesenteries — that is, 

 dou])le the number in ordinary polyp.s — are already developed, though in different examples they 

 exhibit somewhat different relationships. Sixteen complete mesenteries occur, and the remaining 

 eight are incomplete, the paired ai'rangement agreeing with that of simple poh'ps. 



« "The Morphology of the Madreporaria. — II. Increase of Mesenteries in Madrepora beyond the Protocnemic 

 Stage." Ann. Mag. Nat. Hist., ser. 7, vol. x, 1902. 



