476 .MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



On PI. VIII, fig. 64, is given a section through the infolded apical region of Orhicella, in 

 which the filament, and the mesentery to which it is attached, are shown in the act of passing- 

 through an actual perforation of the column wall. The extra-polypal portion of the filament is 

 charged with numerous, large, thin-walled, oval nematocysts, and the disrupted column wall 

 reveals no histological peculiarities. 



The extrusions from coral polyps can scarcely be compared with the ejection of "Acontia," 

 a phenomenon characteristic of the Sagartinse among the Actiniaria. Acontia are thread-like 

 structures, which are but feebly attached to the mesenteries, and pass through permanent aper- 

 tures (cinclides) in the column wall of the polyps, or through the mouth, the mesentery in no 

 ways following. If not wholly liberated from the polyp, the acontium can be indrawn. The 

 extruded filaments of corals, on the other hand, still retain their normal position along the 

 contorted edge of the mesentery, and a portion of the latter passes out along with them. The 

 function of both is probably the same, as in each case the organs are strongly charged with 

 nematocysts. and less so with gland cells. 



ORIGIN OF MESENTERIAL FILAMENTS. 



Probably there is no subject affording greater diversity of opinion among writers on the 

 Anthozoa than that of the ectodermal or endodermal origin of the mesenterial filaments. In the 

 Alcyonaria the problem has been made the subject of special study by E. B. Wilson (188-1): in 

 the Actiniaria, byMcMurrich (1891); while H. V. Wilson (1888) has made it the object of lengthy 

 notice in the coral Ma/nidna. 



The actual facts of the case are briefly as follows: In the adult polyps of all three groups, 

 the stomoda?al ectoderm is in absolute continuity with the mesenterial filaments of the complete 

 mesenteries, as the latter become free at the lower termination of the stomodseum, and the two 

 agree closely enough in their histological detail to suggest a common origin. Further, the fila- 

 ment differs markedly in structure from the mesenterial endoderm. and would thus appear to 

 have no connection with this layer. Likewise in very early larval stages, the same unbroken 

 passage from the stomodwal ectoderm to the filament is often found to exist, though rudiments 

 of the filaments may be present on the primary mesenteries before or independently of their 

 union with the stomodseum. Were the complete mesenteries only to be taken into account, as 

 in the Alcyonaria, the problem would be much simplified, but in both Madreporaria and 

 Actiniaria exactly similar filaments to those on complete mesenteries are found on the incomplete 

 mesenteries, which remain free from the stomodamm. and hence are never in continuity with 

 its ectodermal lining. 



The independence of the filamental and stomodaml tissues within the early larva or bud, 

 along with their histological difference, led E. B. Wilson to regard the six ventral filaments in 

 the Alcyonaria as endodermal; on the other hand, the apparent continuity with the stomodaeal 

 ectoderm of the two dorsal filaments from the beginning, and the closer histological resemblance 

 of the two structures, caused Wilson to regard these as ectodermal. 



From evidence of a like character, McMurrich, in 1891, came to the conclusion that the 

 Drusenstreif or glandular streak on the middle lobe of the Actinian filament is of endodermal 

 origin, while the ciliated bands on the lateral lobes arc ectodermal. Returning to the con- 

 troversy in 1899, McMurrich, from his investigations of the mesenterial filaments in Zodnthws 

 sociatus, again concludes that the ciliated bands must be conceived as ontogenetically distinct from 

 the glandular streaks. Regarding the. ectoderm and endoderm of the Ccelentera as representing 

 but an approximation to the diblastic condition of the higher groups, McMurrich is constrained to 

 regard the distinction between an ectodermal and endodermal origin of any of the organs as 

 of relatively little moment. With this understanding, he concludes; "the ciliated bands are 

 probably in all cases ectodermal; and that in some mesenteries at least, the glandular streaks are 

 endodermal, yet I am prepared to accept as correct the ectodermal origin of the glandular streaks 

 in other mesenteries." The "intermediate" epithelium of the trilobed Actinian filament 

 McMurrich is inclined to regard as ectodermal: my own observations, on species where it is 

 favorably developed for study, lead me to consider it as endodermal. 



