MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 563 



nervous elements; the posterioi' margins of the filament inclose a tissue exactly resembling that 

 of the mesenterial epithelium. 



Traced below, the filament undergoes some change in outline, and two lateral lobes of 

 endoderm are developed to a greater or less extent. This is shown in fig. 57c. The lateral lobes 

 differ but slightly from the ordinary mesenterial epithelium, except that the cells are longer and 

 the nuclei are aggregated toward the free surface. As shown in the particular example figured, 

 the lobes on the two sides are not alwaj^s symmetrical. 



Still lower in the polyp, where a certain degree of convolution takes place, large oval 

 nematocysts predominate in the filament, and gland cells are more numerous (fig. 58). All stages 

 in the development of the large nematoc3^sts can be traced, from one in which the contents are 

 perfectly homogeneous and deeply-staining, with no trace of any thread, to the more mature 

 forms whore the spiral thread is full}- developed. 



The skeletotrophic tissue is narrow throughout, and the three layers — endoderm, mesoglcea, 

 and ectoderm — can usually be distinguished, though not always the last. The endoderm in 

 the upper region differs only slightlj' from its character in other regions of the polyp, being 

 a simple compact columnar layer, the cells of which contain zooxanthellie and much granular 

 matter. The mesoglcea is extremely thin, and rarely distinguishable as more than a dividing 

 lamella between the two cellular laj'ers. The appearance of the ectoderm in the upper, rapidly 

 growing parts of the polyp is represented in figs. 59, 63. The free border is jagged and irregular 

 in outline, as if torn from some other surface with which it was in structural continuity, and the 

 cells form a simple columnar epithelium. Away from the upper edges of the septa, etc., the 

 calicoblast layer becomes much reduced in thickness, and loses its columnar character; generally 

 a few desmoidal processes occur in sections near the insertion of a mesentery. 



Toward the ffoor of the calice the calicoblastic layer is often found in an active condition, 

 and the endoderm over it is much thicker and crowded with coarse granules; in such cases 

 dissepiments are probably in course of formation. 



The gastro-coelomic cavity above is only partlj' subdivided by the mesenterial and septal 

 invaginations (figs. 49, 53), and is continued over the edge of the calice as a narrow space, again 

 subdivided by mesenterial partitions and costal outgrowths (fig. 5-1:). In the lower regions -the 

 central part of the cavitj' ))ecomes more encroached upon by the ingrowth of the septa, and by 

 the great increase in thickness of the endodermal layer covering them (figs. 54, 55). Ultimately 

 the twelve entoccelic septa meet in the middle, and twelve interseptal chambers are formed, 

 wholly cut off from one another, and each pai'tly subdivided by the exocoelic septa which never 

 meet in the middle. 



Genus ORBICELLA Dana." 



Polyps verrucose, distinct, closely united one with another along' a polygonal hase, and forming 

 massive colonies of various shapes, fixed, incrusting, or free, with perithecal continuation of the 

 gastro-coelomic cavity and mesenteries. Column cylindrical, polygonal at the base, on retraction may 

 almost completely fold over the disk; diifuse endodermal sphincter muscle usually present. Tentacles 

 hexamerous, entoccelic and exocoelic, tricyclic or tetracyclic, tuberculated, sometimes swollen toward 

 the origin, rounded at apex, introvertible, cycles close or widely apart. Stomodaeal wall ridged. 



Mesenteries hexamerous, usually di- or tri- cyclic, one or two cycles complete and filamentiferous, 

 two pairs of directives present. Septal invaginations entoccelic and exocoelic, tri- or tetra-cyclic, divid- 

 ing the gastro-coelomic cavity into partly distinct chambers. 



Asexual reproduction by marginal and intercalary gemmation. 



E.XAMPLKs. — <)rl)ici_'U(i (iitni(Iiir!.i< (Ell. & Sol.), 0. radlutu (Ell. & Sol.), O. cavernosa (Linn.). 



""Colony of various shapes, convex, subspherical, subplane, short or tall, adherent, incrusting, or free. 

 Corallites united by continuation of the exotheca, which exists betn-een and beyond the costa' beneath the surface, 

 rarely by tlie costie themselves. Walls usually, but not invariably, stout. Calices with free circular margins, more 

 or less crateriform and elevated. Columella well developed, spongy, and not projecting, with a plain free surface. 

 Septa exsert or not, with well-developed laminn?, thicker near the margins tlian near the columella, where there is 

 often a paliform tooth, dentate. Costa' well developed, passing over tlie surface for some distance; where seen on the 

 wall of a t'orullite they are lamellar and well developed, often spiny. Endotheca well developed. Exotheca between 

 and usually beyond the costa', well developed. Eiiitheca may or may not exist. Gemmation inframarginal, and 

 from the area between the calices." {Helioitra'd , Duncan, 188.5, p. 104.) 



