MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 479 



mesentery is. fully formed (fig. 57 c). If the filaments of the incomplete mesenteries originate 

 from ;i reflected tract of stomodseal ectoderm, it might reasonably be expected that they would be 

 best developed in the uppermost region of the polyp, whereas, as a matter of fact, they arc here 

 absent or only incipient, even when fully developed below. 



All the evidence seems to favor the view that in the Madreporaria the mesenterial filaments 

 first appear independently of any connection with the ectodermal lining of the stomodseum, but 

 that in the case of the complete mesenteries such a continuity is early established, while with 

 incomplete mesenteries the separation is permanent. 



When describing the adult mesenteries, it is shown that the first part of the filament differs 

 in form and structure from that below, and histologically is indistinguishable from the stomodseal 

 ectoderm, with which it is in direct continuity. It is manifest, therefore, that between the 

 actual stomodseal termination and the commencement of the true filament there is a tract which 

 partakes more of the nature of the stomodseal ectoderm than of the filamental tissue. It forms 

 the connecting link, as it were, between two tissues which may be considered as wholly distinct 

 both phylogenetically and ontogenetically. Such, it may be conceived, is the significance of the 

 "reflected ectoderm" as seen in corals. It is the stomodseal ectoderm passing along the mesentery 

 to establish structural continuity with the upwardly growing filament. 



The strongly ciliated character of the stomodseal ectoderm marks out the layer as specially 

 concerned in the circulation phenomena of the polyp, and the same must be affirmed to a less degree 

 of the upper part of the mesenterial filament. For the proper carrying out of this it is manifest 

 that a close histological continuity should be maintained between the two structures, and it can 

 be conceived that in establishing this the stomodseal ectoderm passes some distance down or 

 along the mesenterial edge to meet the upgrowing filament proper. 



The mesenterial filaments on the incomplete mesenteries are the homologue of those on the 

 complete filament, but the latter are in more or less direct histological continuity with the 

 stomodseal ectoderm by a downgrowth from the latter, while the others are free throughout. 



In his recent paper (1900., p. 73), Appellof contends for the ectodermal origin of the whole 

 Actinian filament, his fig. 25 showing a distinct passage of the stomodseal ectoderm down the free 

 edge of the mesentery. The occurrence of an interval of endoderm between the stomoda?al 

 ectoderm and early filament, such as McMurrich (1891) and I (1899) have found in other larva?, 

 Appellof would explain as the result of a more or less accidental severance of continuity upon 

 retraction of the larvse. Such a suggestion would scarcely be applicable to the conditions 

 already described in the larva? of Furl,/, for the first indications of the filamental tissue occur at 

 very different levels on the various mesenteries, in some instances at a considerable distance 

 below the stomodseal termination. On the view presented above, the appearances which Appellof 

 describes in Urticiiia are not directly concerned with the formation of the filament; the downward 

 growth of the ectoderm from the stomodseal wall is merely the means of establishing continuity 

 with the true filament which will appear independently below. 



BASAL DISK, SKELETOTROPHIC OR SKELETOGENIC TISSUES. 



Under these terms will be included the, three Ccelenterate layers — ectoderm, mesoghea. and 

 endoderm — which everywhere cover the surface of the skeleton in the living portion of a coral. 

 G. von Koch (1882) has fully demonstrated that the ectoderm alone is the true skeletogenic layer, 

 and is actually adherent to the corallum; but it is convenient to study along with it the associated 

 mesogloea and endoderm. The early stage- in the growth of the young polyp after fixation 

 (PI. XIX) reveal that the basal disk only is concerned in the formation of the skeleton, and 

 therefore all the subsequent foldings, invaginations, and evaginations of the skeletotrophic 

 tissues are but so many extensions of this region of the poly}), produced pari passu with the 

 deposition of the calcareous particles. The theca, septa, costse, columella, and all the teeth, 

 spines, etc.. connected with them, represent so many foldings of the basal disk, for all take their 

 origin from the same continuous layer, and their surface remains covered by it so long as they 

 belong to the living parts of the polyp. 



