562 MEMOIRS OF THE NATIONAL ACADEMY OF .SCIENCES. 



Six pairs of mesenteries, constituting the first order, are perfect, and of these, two pairs, 

 situated one at each end of the longer axis of the stomodseum, are directives (fig. 49); in serial 

 transverse sections these are the first to cease their connection with the stomodseum (fig. 51). 

 The second cycle of mesenteries also consists of six alternating pairs, which are of moderate 

 length in transverse sections, hut never reach the stomodseum. For some distance below the 

 stomodseum little difference exists in the radial length of the mesenteries of the two cycles, but 

 toward the lower region the second-cycle members lose their filaments, and disappear in advance 

 of the others (tigs. ">4. 55). 



Members of a third cycle of mesenteries usually occur, but instead of consisting of twelve 

 pairs, that is. a pair in the exoccele between each pair of mesenteries of the first and second 

 order, only two to six pairs are developed, all situated on the sulcular or sulcar aspect, as the 

 case may be. in regard to the members of the first order (tig. 49). Often the two mesenteries 

 forming a pair are not of equal length in transverse sections, nor do they extend for the same 

 distance along the length of the polyp. All the members of the third order extend but a very 

 short distance below the stomodseal region. The mesenteries of the two first cycles bear fila- 

 ments of exactly similar structure, but they are rarely fully developed on the members of the 

 incomplete third cycle. 



The mesenterial plan of an irregular polyp, having three pairs of directives, is represented 

 in tig. 53; probably it represents a stage toward fissiparous gemmation. 



When free the mesenteries are somewhat clavate in form in transverse section, very narrow 

 at their origin in the body wall, and broadening a little until they terminate in the enlargement 

 of the filaments. The mesoghca is narrow at its origin, and widens beyond, narrowing some- 

 what again toward its free end. and then enlarging at the base of the filament. It is a clear, 

 homogeneous layer without cellular contents, and the face bearing the retractor muscle is 

 thrown into deep, simple or slightly complicated folds, nearly alike throughout its radial 

 length, but a little more pronounced in the middle. The opposite face bearing the oblique 

 musculature is smooth (tig. 59). 



The retractor muscle extends as a simple layer of vertical muscular fibers over the whole 

 surface of one mesenterial face, and the musculature on the opposite face can be readily dis- 

 tinguished in slightly oblique sections. Below the stomodseum the musculature is equally 

 developed on each face of the mesentery for some little distance from the origin, and the fillers 

 on both faces are nearly vertical in direction. The muscle fibers can be traced for a short 

 distance along the body wall, continuous with those from the mesentery, recalling the parieto-basilar 

 muscles of the Actiniaria. As the mesenteries become shorter transversely, and lose their fila- 

 ments, the musculature is of the same character all the way round. 



As shown in tig. 51, the terminal portion of the stomodseal wall becomes reflected outwardly. 

 so that in transverse sections it is cut through twice. The ectoderm narrows somewhat, and passes 

 for some distance along the two faces of a mesentery, swelling a little and then terminating 

 rather abruptly. Traced section by section each mesentery ultimately severs its connection 

 with the stomodseum, but is still capped by the same deeply -staining tissue; thus absolute 

 continuity is made between the stomodseal ectoderm and the mesenterial filaments, and for some 

 distance the histological elements are much the same in both. In the section through the 

 terminal stomodseal region, represented in tig. 51, all the stages in the separation of the six pairs 

 of mesenteries can be observed, the directives being the first to become free. 



A transverse section through a filament, just after the mesentery becomes free from the 

 stomodseum, is represented in tig. .">7/<. The outline for a short distance is cordate, and histologically 

 the filament is constituted of ciliated supporting cells, which are somewhat longer toward the apex 

 than laterally. Long narrow nematocysts occur in this region of the polyp, mostly aggregated 

 about the apex, as well as granular eland cells, especially noticeable toward the internal limits. 

 The mesoglcea from the mesentery passes toward the middle of the filaments, enlarges somewhat, 

 ami then sends a branch to each side. The musculature can in most cases be distinguished on 

 both laces of the mesoglcea of the mesentery, thence passing to the concave surface of the 

 filamental portion. A punctate appearance isalso presented around the convex border, indicating 



