472 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



jrraduiillv into it. F'igs. 14, 39. 44, 4.5, 57, and 09 will give .some idea of tlio varietj' of form 

 presented in triinsvers(> sections. 



Histologitiilly. 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 stomodreal ectoderm and the mesen- 

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

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

 distance along both sides of the mesenteries, and as the latter liecome free thcA' are capped with 

 the ectoderm. In transverse sections (fig. 57^') the filament at tirst is cordate, the mcsogloea 

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

 differ in no respect histologically from the stomod»al ectoderm, while the posterior borders 

 are ordinarj' 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 ionn {C7adocora, Astrangia) than in others. 

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

 the stomodieum. It passes gradually into the ne.xt stage, which represents the longest part of 

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

 of nirrow nematocysts, 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 mesogloja 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 mcsoglceal expansion nervous elements are usuallj' 

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

 mesoglteal border, as a continuation of the mesenterial muscle layer {ef. also figs. 44, 45, PI. V.) 



Passing to the lower regions of the polyp, tli(> filaments usually liecome broader in transverse 

 sections, and large, oval, thin-walled nematocy.sts, 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 (PI. VII. fig. 

 58; PI. XIII. tig. !)4)". 



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

 confounded with the two lateral lobes of the trilobed Actinian filament. In these the three 

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

 a separate mesoglteal axis. The apical part of the middle lol)e (Drusenstreif) is mainly glandular 

 in character in the upper region of the polyp, and a few small nematocysts usualh' 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 clo.se re.seml)lance 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 lo])es. The lateral lobes of coral pol^yps never contain a 

 separate mesogheal axis, and histologically they bear the close.st resemblance to the ordinary 

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

 tallv on the incomplete mesenteries, while in certain genera {('(tri/iiorf/K, Ji/iodaet/.s) 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 tilaiuent. especially in Miuh'i pant 

 (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 

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



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

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

 fig. SV. but in the polyps of certain Pacific corals I have observed the phenomenon noticed liy these authors. 



