562 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Six pairs of niesentei'ies, constituting the first order, are perfect, and of these, two pairs, 

 situated one at each end of the longer axis of the stonioda'uni. are directives (fig. -ilt): in serial 

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

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

 length in transvoi'se sections, hut never reach the stomoda'um; For some distance ))elow the 

 .stomodanuii little difierence exists in the radial length of the mesenteries of the two cycles, l)ut 

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

 of the others (figs. 54, 55). 



Menil)ers of a third cycle of mesenteries usually occur. ))ut instead of consisting of twelve 

 pairs, that is, a paii- 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 lie. in regard to the members of the first order (fig. 49). Often the two mesenteries 

 forming a pair are not of cipial 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 stomoda-al region. The uiesenteries of the two first cycles l)ear fila- 

 ments of exactly similar structure. ))ut they are rarely fully developed on the members of tiie 

 incomplete third cycle. 



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

 in fig. 53; proliably it represents a stage toward fissiparous gemmation. 



When free the mesenteries are somewhat clavate in form in transverse section, very nari'ow 

 at their origin in the body wall, and l)roadening a little until th(>y terminate in the enlaigcment 

 of the filaments. The mesoglcea 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 nuiscle is 

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

 length, but a little more pronoiuiced in the middle. The opposite face bearing the (>l)li(|ue 

 musculature is smooth (fig. 59). 



The retractor nuiscle 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- 

 tingiushed in slightly oblique sections. Below the stonK)da'um the musculatui'e is equalh' 

 developed on each face of the mesenterj' for some little distance from the origin, and the fibers 

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

 distance along the l)ody wall, contiiuious with those from the mesentery, recallingthe parieto-l)asilar 

 muscles of the Actiniaria. As the mesenteries ])ecome shorter transversely, and lose tlK'ir fila- 

 ments, the nuisculature is of the same character :dl the way round. 



As shown in fig. 51, the terminal portion of the stomoda'al 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 ultimateh' severs its connection 

 with the stomoda?um, but is still capped l)y the same deeply -staining tissue; thus absolute 

 continuity is made between the stomoda>al ectoderm and the mesenterial filaments, and for some 

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

 terminal stomoda?al region, represented in fig. 51, all the stages in the sejjaration of the six ]>airs 

 of mesenteries can be observed, the directives 1)eing the first to become free. 



A transverse section thi-ough a filament, just after the mesentery becomes free from tiie 

 stomodfBum, is represented in fig. 57/>. The outline for a short distance is cordate, and histologically 

 the filament is constituted of ciliat(Hl supporting cells, which are somewhat longer toward the apex 

 than lateralh'. Long narrow neinatocysts occur in this region of the polyp, mostly aggregated 

 al)out the apex, as well as granular gland cells, especially noticeable toward the internal limits. 

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

 and then sends a l)ranch to each side. The musculature can in most cases be distinguished on 

 both faces of the mesoghea of the mesentery, thence passing to the concave surface of the 

 filaniental portion. A j)unctate appearance is also presented around the convex border, indicating 



