MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 471 



longer and shorter mesenteries (anisocnemic), with the exception of the directives, which, whether 

 lateral or axial, consist of equal moieties (isocnemic). 



5. When the number of mesenteries in a polyp reaches twelve pairs, stomodseal fission may 

 take place, in such a manner that six primary and six new mesenteries are associated with each 

 stomodseum. 



A great distinction is thus established between the manner of appearance of the metacnemes 

 in Poriti s and that in other coral polyps. In the former, the additions are shown to take place in 

 bilateral pairs at only one region, and within an entocoele, while in the latter it has been shown 

 that the additions are made in unilateral pairs all round the polyps, within the six primary 

 exoccelic chambers. In Porites, the unilateral pairs consist of a larger and a smaller moiety 

 (anisocnemic), without the formation of hexameral cycles, while in other Madreporaria the 

 members of a pair are alike in size throughout (isocnemic), and in the end the different pairs 

 constitute cycles. 



Later results suggest that the additions in Poritv* are to be regarded as stages in the process 

 of fissiparous gemmation. (See foot-note. p. 496.) 



INCREASE OF MESENTERIES IX MADREPORA. 



In a recently published paper," I have fully described the peculiar manner in which the 

 increase of mesenteries beyond the protocnemic stage takes place in Mad/repora. The process 

 is again alluded to on p. 515, in connection with fission in Madrepora, and is illustrated by three 

 diagrammatic figures (tig. ISa-e). Fundamentally, the increase takes place in the same manner 

 as in Porites, that is, by bilateral pairs, which are disposed within the directive axial entocoele. 

 But in any one polyp of Madrepora additions are made at both extremities, whereas, in Porites, 

 they are restricted in any one polyp to either the dorsal or the ventral directive entocoele. 

 Six new pairs seem to rise simultaneously in Mad/repora, as against the successive order in 

 Porites; for some time one or two of the pairs may be united with the two stomodseal tubes, 

 without any connection with the column wall. 



MESENTERIAL FILAMENTS. 



The edges of all the complete mesenteries, after ceasing their connection with the 

 stomodseum, are provided with the Anthozoan structures known as mesenterial or gastric 

 filaments, and likewise the free edge of most of the other mesenteries, which at no time extend 

 transversely so as to reach the stomodseum. In dissected polyps the filaments appear as 

 dense, white, thread-like organs, connected with the mesenteries, usually straight and vertically 

 descending in the upper region, but greatly convoluted below. In the living condition, they are 

 frequently extruded through the mouth and polypal wall, as white coiled threads, along with the 

 portion of the mesentery to which they are attached. They are generally strongly marked off 

 from the rest of the tissues in microscopic preparations, on account of the brightly-staining 

 character of their cellular constituents. 



On the incomplete mesenteries, the filament-, as a rule, commence a short distance from the 

 uppermost region of the polyp, and terminate below somewhat in advance of the mesentery: 

 occasionally they are absent from the last cycle of mesenteries, or remain incipient. On 

 the other hand, the filaments are borne by the complete mesenteries only after ceasing their 

 connection with the stomodseum. At first they are straight, but soon become greatly 

 convoluted, <>u sonic mesenteries more than on others. 



Structurally, the filaments display the same essential characters in all the species examined, 

 and are simpler than the corresponding organs in the majority of Actiniaria. Throughout the 

 .Madreporaria, so far as yet known, the actual filament consists of only a single median lobe, in 

 contrast with the trilobed condition of most Actinian filaments. In transverse sections the 

 organs appear as cordate or disk-like expansions of the edge of the mesenteries. In addition 

 to the actual terminal filament, the mesenterial epithelium immediately behind is usually much 

 swollen on each side, and is either sharply rounded off from the rest of the endoderm or passes 



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

 Stage." Ann. Mag. Nat. Hist., Ser. 7, Vol. X, 1902. 



