590 MEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 



internall}' at about equal distances apart, and the backwardly directed, free edge of the stoniiHlanini 

 passes outwardly for sonic distance aioni;' tiieir faces, and is continuous with the mesenterial 

 tilaments. A few neniatocvsts are foiuid in the stoniodival ectoderm, and louu- narrow alaiui c(dls 

 toward the outer part of the layer. The me.soglcea is extremely delicate, while the endoderm is 

 slightly broader tlian tiiat of the colunm wall. 



Three orders of mescMiteries occur. The members of the tiist ordei- reach tiie stomoda'um: 

 the secondary pairs ma}' extend centrally nearly as far as the stomodanun; while those of the 

 third cycle are some distance away, but are nevertheless well developed (tig. 153). Apparently 

 the complete condition should be 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 thii'd cvcle — twenty-four pairs in all. This regularity, however, is not attained in any of 

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

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

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

 reached so far; in another specimen the two pairs of directives were normally developed. 

 Usuallv one or more of the pairs necessary to complete the twelve pairs of the outermost, tliird 

 cj'cle are wanting; rarely one or more pairs of a fourth cycle are present. 



Except in the u])pcrmost region, each interseptal locidus appears broken up into separate 

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

 peripheral lioundai'v of the polyp only within the uppermost stomoda'al region; below this 

 region the interseptal loculi are devoid of any contents in their peripheral chamliers. the 

 mesenteries having wholly disappeared. In some cases the mesenteries maj^ extend across two 

 chambers, as seen in transverse sections, l)ut rarely more; in vertical pei'ipheral sections traces 

 can sometimes be found extending through three or four I'ows of synapticula (tig. 15(i). 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 poly])s. 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 nuiscles of the mesenteries are comparatively well 

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

 (fig. 15S); in favorable sections the oblique musculature on the smooth face of the mesentery is 

 also distinguished. The mesenterial epithelium contains numbers of zooxanthella; 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 skeletotrophic tissues. 



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

 second and third cj'cles, which never reach the stomoda?um. In the stomodajal region the 

 incomplete mesenteries exhibit only the earliest stages in tilaniental 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 bec-ome rounded, nematocysts and deeply-staining gland cells 

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

 in some cases, as in fig. 159, no endodermal swelling occurs. In the aboral region the filament 

 disappears some distiince in advance of the mesentery. 



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

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

 invaginations (fig. 153). The interseptal loculi are very narrow, and where the larger septa meet 

 in the middle some of the interseptal chambers are wholly cut oft' fi'om one another, each partly- 

 subdivided peripherally bv 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 



