310 DR. J. E. DUEEDEN ON THE 



be discerned with difficulty. Near its insertion in the column-wall the mesentery 

 is usually a little broader, and the retractor muscle is represented as a smooth layer of 

 fibrils. In other polyps the mesenteries are thicker, and muscle plaits are very distinct 

 in the middle of their transverse length. Belon^ the stomodseal region all the 

 mesenteries become much broader, both the endoderm and mesoglcea increasing in 

 thickness, and the latter forms numerous plaitings for the support of the retractor 

 muscles {ret.m., PI. 26. figs. 16 & 17). The retractor muscle in fact assumes somewhat of 

 a circumscribed character, tnit the outline in transverse section is scarcely alike in any 

 tAvo mesenteries. Por a short distance from the column-wall the muscle-band is smooth ; 

 the plaitings beyond are at first minute, then gradually become larger and more 

 complicated, and terminate very abruptly, the mesentery afterwards being very narrow 

 and continued for some distance (fig. 10). At the central termination of the complicated 

 part of the muscle one or more of the mesogloeal plaitings may become greatly elongated 

 and minutely plaited, almost as if the mesentery w^ere about to branch (fig. 17, ret.m.). 



The transverse musculature is comparatively strongly developed. In tangential 

 sections near the base the mesogloea of the mesentery is irregularly folded for its support 

 (fi-g. 2, trans.m.). When a complete mesentery is mounted as a microscopic object, the 

 vertical and transverse muscle-tiljrils are seen crossing one another at right angles. 



The mesenterial endoderm is usually very narrow, and includes clear gland-cells and but 

 few Zooxanthellse. Traces of the nerve-layer, continuous with that on the column-wall, 

 can usually be followed (nr.l., fig. 16) for some distance beyond the insertion of the 

 mesenterv in the column-wall. In the lower region the mesenterial epithelium becomes 

 swollen just behind the mesenterial filaments [cf. figs. 16 & 17), and in some polyps this 

 region contains many clear spheroidal bodies, w^hich stain very deeply and stand out as 

 distinct objects from the rest of the tissue ; a few small particles are black and 

 irregularly shaped. Probably these foreign bodies are ingested nutrient particles, and 

 they are practically absent from some individuals. Where present in quantity they may 

 occur also in the endodermal epithelium of the lower part of the stomodseum, and 

 sparsely in the endoderm of the column-wall, and even in the tentacular endodermal 

 epithelium. The particles are easily distinguished by their homogeneous, deeply staining 

 character from the Zooxanthellse, which they greatly resemble in size and outline. 



The swollen mesenterial endoderm immediately behind the mesenterial filament is 

 generally regarded as a special region for intracellular digestion (McMurrich, 1899, 

 p. 262), and in Btowdeopsis the greatest number of foreign particles occurs there ; but 

 their presence in other regions of the endoderm. would imply that ingestion is by no means 

 restricted to the region. The endodermal enlargements in the present species are often 

 somewhat irregulai- iu outline (fig. 17), not smoothly rounded as in most other forms. 



At the origin of the mesenteries in the column-wall a delicate layer of muscle-fibrils, 

 arranged nearly vertically, occurs on the face bearing the transverse musculature. This 

 is the only trace of a parieto-basilar muscle ; in no instance does the mesoglcea become 

 plaited or afford any indication of the formation of a pennon, such as is characteristic 

 of Actinian species with a well-developed muscle. 



