472 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



gradually into it. Figs. 14, 39, 44, 45, 57, and G9 will give some idea of the variety of form 

 presented in transverse sections. 



Histologically, a filament differs in passing vertically from one region of the polyp to another, 

 and also exhibits a variety of cellular constituents in different parts of the same section. In 

 the complete mesenteries the actual boundary between the stomodseal ectoderm and the mesen- 

 terial filament is by no means well defined. As shown on PI. VI. tig. 51, the deeply-staining 

 ectoderm of the stomodseum, at the termination of the latter, appears to pass around and for some 

 distance along both sides of the mesenteries, and as the latter become free they are capped with 

 the ectoderm. In transverse sections (rig. 575) the filament at first is cordate, the mesoglcea 

 bifurcating and supporting the lateral wings. The anterior and lateral borders of the filament 

 differ in no respect histologically from the stomoda^al ectoderm, while the posterior borders 

 are ordinary mesenterial endoderm. The first part of the filament in most corals is of this charac- 

 ter, but continues thus for a longer distance in some form (Cladocora, Astrangid) than in others. 

 The stage is never represented in the filaments of mesenteries which are unconnected with 

 the stomodicum. It passes gradually into the next stage, which represents the longest part of 

 the filament (fig. 57c). The anterior or inner portion of the filament is constituted mostly 

 of narrow nematoeysts, clear and granular gland cells, and supporting cells, while laterally and 

 behind the cells become shorter, supporting cells predominate, and the ciliation is stronger 

 than elsewhere. The mesoglcea of the mesentery passes but a short distance into the filament, 

 and there bifurcates, each half being directed forward, horizontally, or backward, and quickly 

 thinning out. Immediately in front of the mesogheal expansion nervous elements are usually 

 recognizable, and less often muscular fibrils. The latter may also be present along the hinder 

 mesoglceal border, as a continuation of the mesenterial muscle layer (cf, also figs. 44, 45, PI. V.) 



Passing to the lower regions of the polyp, the filaments usually become broader in transverse 

 sections, and large, oval, thin-walled nematoeysts, with a very distinct spiral thread, are the chief 

 feature. So abundant are the stinging cells that in some cases they make up by far the greater 

 proportion of the whole filament, the supporting cells serving as a kind of matrix (PL VII, fig. 

 58; PI. XIII, fig. !>4)". 



The swollen mesenterial endoderm, immediately behind the filament, must in no ways he 

 confounded with the two latei'al lobes of the trilobed Actinian filament. In these the three 

 lobes are very distinct structures, both as to their form and histology, and each is supported upon 

 a separate mesoglcea] axis. The apical part of the middle lobe (Drusenstreif) is mainly glandular 

 in character in the upper region of the polyp, and a few small nematoeysts usually occur. On 

 their antero-lateral borders, the two lateral lobes are constituted wholly of ciliated supporting 

 cells, being known as the ciliated bands or Flimmerstreifen. Between the glandular streak 

 and the ciliated bands is found a patch of tissue, which as a rule bears a close resemblance to 

 undifferentiated endodermal epithelium, and has been termed the intermediate streak. 



Comparing the coral filament with that of the Actinian, it is manifest that the organ in the 

 former is represented by the middle lobe of the latter, and there is nothing which corresponds 

 morphologically with the lateral lobes. The lateral lobes of coral polyps never contain a 

 separate mesoglceal axis, and histologically they bear the closest resemblance to the ordinary 

 mesogloeal epithelium. In Actinians the lateral ciliated lobes disappear aborally, and also dis- 

 tallv on the incomplete mesenteries, while in certain genera (Corynactis, Rhodactis) the lateral 

 lobes are altogether wanting, when the filament is essentially like that of the Madreporarian 

 polyp. 



Histologically the postero-lateral region of the coral filament, especially in Madrepora 

 (p. 474), closely recalls the ciliated streak of anemones, and its .strong ciliation also suggests a 

 similar function. 



The filaments on the imperfect mesenteries often remain in a rudimentary condition, and 

 afford instructive stages in the development of the organ. The free edge of the mesentery is 



"None of the mesenterial filaments examined ever show the nematoeysts partly extruded, in the manner 

 described ami figured by Bourne for Fungia (1893, pi. XXIV. fig. 28), and by Pratt for Neohelia i 1900, pi. LXIII, 

 li'_r. si; 1 nit in the polyps <>f certain Pacific corals 1 have observed the phenomenon noticed by these authors. 



