590 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



internally at about equal distances apart, and the backwardly directed, free edge of the stomodseum 

 passes outwardly for some distance along their faces, and is continuous with the mesenterial 

 filaments. A few nematocysts are found in the stomodseal ectoderm, and long narrow gland cells 

 toward the outer part of the layer. Tin 1 mesoglcea i^ extremely delicate, while the endoderm is 

 slightly broader than that of the column wall. 



Three orders of mesenteries occur. The members of the first order reach the stomodseum; 

 the secondary pairs may extend centrally nearly as far as the stomodseum; while those of the 

 third cycle are some distance away, hut are nevertheless Midi developed (fig. 153). Apparently 

 the complete condition should lie six pairs of perfect mesenteries, two pairs of which are 

 directives; six alternating pairs constituting the second cycle; and twelve alternating pairs making 

 up the third cycle — twenty-four pairs in all. This regularity, however, is not attained in any of 

 the polyps sectionized transversely. In two examples only one pair of directives occurs, the 

 corresponding axial pair having the retractor muscles on the faces turned toward one another. 

 Eleven mesenteries extended as far as the stomodseum in one polyp, while the twelfth never 

 retu-hed so far; in another specimen the two pairs of directives were normally developed. 

 Usually one or more of the pairs necessary to complete the twelve pairs of the outermost, third 

 cycle are wanting; rarely one or more pairs of a fourth cycle are pi - esent. 



Except in the uppermost region, each interseptal loculus appears broken up into separate 

 chambers, as a result of the presence of synapticula. The mesenteries extend as far as the 

 peripheral boundary of the polyp only within the uppermost stomodseal region; below this 

 region the interseptal loculi are devoid of any contents in their peripheral chambers, the 

 mesenteries having wholly disappeared. In some eases the mesenteries may extend across two 

 chambers, as seen in transverse sections, but rarely more; in vertical peripheral sections traces 

 can sometimes be found extending through three or four rows of synapticula (tig. 15r>). The 

 manner of disintegration and resorption of the peripheral and aboral areas of the mesenteries, 

 as they become perforated by the synapticular growths, has been already described (p. 4s7). 

 Centrally some of the mesenteries extend more than halfway down the length of the polyps, but 

 none reach the aboral termination, and all are much shorter peripherally; in the middle part of 

 their course they become somewhat convoluted. 



In the upper region the retractor muscles of the mesenteries are comparatively well 

 developed, arranged on slight foldings of the mesoglcea which extend nearly across the face 

 (tig. 158); in favorable sections the oblique musculature on the smooth face of the mesentery is 

 also distinguished. The mesenterial epithelium contains numbers of zooxanthelhe and irregular, 

 highly refractive granules. Sometimes these latter occur singly, at other times in groups, or 

 even in rounded masses; they seem to be inclosed in vacuoles, and are perhaps products of 

 digestion. The granules seem more numerous where the disorganization of the mesenteries 

 is taking place, so that probably the products of this activity are absorbed by the more centripetal 

 mesenterial epithelium, as well as by the endoderm of the skeletotrophie tissues. 



Fully developed mesenterial filaments occur on all the mesenteries, including those of the 

 second and third cycles, which never reach the stomodseum. In the stomodseal region the 

 incomplete mesenteries exhibit only the earliest stages in filamental development; the tissue 

 at the free end stains more deeply than the rest of the epithelium, but is not swollen (tig. 158). 

 Lower, however, the filaments become rounded, nematocysts and deeply -staining gland cells 

 occur, and the mesenterial epithelium immediately behind is usually swollen and rounded oil; 

 in some cases, as in fig. IT)!*, no endodermal swelling occurs. In the aboral region the filament 

 disappears some distance in advance of the mesentery. 



In retracted polyps the gastro-ccelomic cavity above is divided centrally only by the 

 mesenterial partitions, but in the peripheral portion it is subdivided in addition by the septal 

 invaginations (tie-. L53). The interseptal loculi are very narrow, and where the larger septa meet 

 in the middle some of the interseptal chambers are wholly cut off from one another, each partly 

 subdivided peripherally by the shorter exoccelic invaginations. Owing to the presence of 

 synapticula, and the union of the septa with one another centrally, the polypal cavity in sections 

 appears greatly subdivided and intruded upon. The individual interseptal chambers never 



