MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 489 



All stages in the resorption of the lower parts of the mesenteries can be observed. ^Vhere 

 the action i.s in progress the peripheral edge is free and tapering, although the mesentery was 

 originally attached by this to the wall (fig. 158). The mesogloea is seen to break up into distinct 

 pieces, and the endodermal epithelium is in diiierent stages of disorganization; tei'ininal 

 fragments appear as if about to break ofl, and occasionally free particles are met with. The 

 mesenterial debris thus set free i.s evidenth' injested by the endodermal epithelium lining the 

 chambers, for the layer is here of exceptional thickness and the large cells are crowded with 

 granules and irregular fragments, which closely recall those given off from the disintegrating 

 mesentery. 



Fig. 1.53, taken from a retracted polyp, reveals that the mesenteries do not extend as far as 

 the most perijjheral chaml)ers, although the region rei^resentcd is no lower than the stomodteum. 

 In some cases a fragment of the mesentery may persist in the second chamber, but its imperfect 

 character indicates that it is about to disappear; even where the section does not actually 

 encounter a synapticular interruption the peripheral tissue is atrophied. 



In the tangential section, fig. 156, the mesenteries all extend vertically beyond the first 

 transverse rows of synapticula. but in the chambers below they begin to exhibit the various 

 stages in absorption. 



That the mesenteries are actually pierced liy the synapticular formations is manifest from 

 the preparations. AVhen serial sections are passed in review, it is seen that the mesentery 

 whoUj' surrounds the upper and more central perforations left by the removal of the synapticula, 

 and frequently the mesenterial me.sogloea becomes swollen, and presents .striated areas, such as 

 are formed l)y the desmocytes where a mesentery is inserted on the calicular wall (fig. 157). 



Miss Ogilvie has attributed an altogether different origin to the .S3'naijticula, in her account 

 of these structures in Fmiyia and Siderastreea. Commenting (jj. 170) upon Bourne's description 

 of the synapticula in Fungia. she states: "The important point is that they neitlwr ' inttrr\qiV 

 nor '^^V/'w' tlie mesenteries.'''' Further, it is a.ssumed all along that the l)ody wall is specially 

 invaginated from below to produce them". Had an examination of tlie actual polj'pal tissues 

 been made it is impossil)le to see how any support could have been adduced for such statements, 

 any more than would be forthcoming for the production of simple tubercles on the septa. 



Professor Bourne, in his paper, "The Anatomy of the Madreporarian coral Fungicr'' (1887), 

 also describes somewhat similar mesenterial relationships in the genus Fnngia, only here the 

 synapticula are in .single vertical or oblique bars, not in Aertical rows, as in Siderantrsea. In the 

 upper regions of the interseptal chambers there are no synapticula, and the mesenteries are free 

 to radiate across the whole space between the stoniod;eum and the periphery of the disk, but in 

 the lower portions of the loculi the continuity of the mesenteries becomes interrujited In' 

 the sj'napticula. Owing to the much larger number of vertical bars across the broad .septa 

 of Fiing'iii^ the intersynapticular cavities in sections greatly outnumber those of Slderastra'a, 

 and the mesenteries do not extend wholly across any .segment, being represented by a small 

 projection at each extremity of the chamlier (Bourne's figs. 13, 15). Bourne's explanation 

 (1887, p. 19) of the significance of the .synapticula, that "physiologically the\^ seem to .serve _ 

 as stays or buttresses, giving solidity and coherence to the coralluni." is probably the most 

 correct of any yet offered. From the disappearance of the mesenteries below, almost ^;»ari^(7.<s?< 

 with th(> d(>vi'Iopment of the synapticula. the circulation of the digestive fluids and functional 

 activity witliin the .synapticular region becomes diminished, and it is very doubtful if. as ]\Iiss 

 Ogilvie (p. 171) .suggests, the main advantage is that "an increased endodermal surface is 

 afforded within the visceral cavity." 



"Acting upon thi.s suggestion of Miss Ogilvie, Delage and Herouanl, in tlieir "Traito ile Zoologie Concrete, Tome 

 II, pt. 2, Le.s Ciflenteres," 1901, liave constructed two ingenious diagrammatic figures (pi. 62, figs. 1, 2), attempting 

 to show Ikjw the ba.'iUl infolding of the soft wall of the polyp proceeds in the formation of Ijoth bar-like and lamellar 

 synapticula. The polyps of Sideraxlrwa give no support whatever for such a conception. From the interseptal 

 lamella rejjre.^ented on PI. XXII, fig. 1.52, it is manifest that each synapticulum i.s formed independently of the 

 others, not from a continuous infolding of the hasal part of the skeletogeuic layer, as Ogilvie and Delage & Ilcrouard 

 assume. 



