MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 563 



nervous elements; the posterior 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 bul 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 always 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 nematocysts can he traced, from one in which the contents are 

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

 forms where the spiral thread is fully developed. 



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

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

 the upper region differs only slightly 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 mesogkea is extremely thin, and rarely distinguishable as more than a dividing 

 lamella between the two cellular layers. The appearance of the ectoderm in the upper, rapidly 

 growing parts of the polyp is represented in figs. 59, «'>:'>. 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 

 calieohlast 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 floor 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 partly 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. 54). In the lower regions the 

 central part of the cavity becomes 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, 

 whollv cut off from one another, and each partly subdivided by the exocoelic septa which never 

 meet in the middle. 



Genus ORBICELLA Dana." 



Polyps verrucose, distinct, closely united one with another along 1 a polygonal base, 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; diffuse 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-ccelomic cavity into partly distinct chambers. 



Asexual reproduction by marginal and intercalary gemmation. 



Examples. Orbieella annularis (Ell. & Sol.). 'A radiata (Ell. & Sol.). (>. cavernosa (Linn.). 



"Colony of various shapes, convex, Bubspherical, subplane, short or tall, adherent, incrusting, or tree. 

 Corallites united by continuation of t lit- exotheca, which exists between and beyond the costss beneath the surface, 

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



. . r less crateriform ami elevated. Columella well developed, spongy, and not projecting, with a plain free surf: 



Septa exsert or not, with well-developed lamina', thicker near the margins than near the columella, where there is 

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

 wall of a coral lite they are lamellar and well developed, often spiny. Endotheca well developed. Kxotheea between 

 and usually beyond the costa?, well developed. Epitheca mayor may not exist. Gemmation inframarginal, and 

 from the area between the calices." [Heliattrsea, Duncan, L885, p. 104. 



