434 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Histologically the disk differs but little from the tentacles or column wall. The peristome 

 is generally very thin walled, and in nearly all cases is provided with weak radiating ectodermal 

 and circular endodermal musculatures. Granular and clear gland cells are generally numerous, 

 and nematocysts, somewhat similar to those in tin 1 tentacles, are sparingly distributed. In some 

 cage's a delicate ectodermal nerve layer can also be distinguished, but is never so pronounced 

 as in the tentacles. 



In fully retracted polyps of many species the discal wall is found partly introverted within 

 the polypal cavity, somewhat in the same manner as already described for the tentacles (p. 4:23). 

 Such a condition is often very confusing during the study of sections, and may seriously 

 interfere with the determination of the relationship of the mesenteries to the stomodaeum. In 

 one important respect the invagination is distinguishable from the introversion of the tentacles: 

 it is essentially mesenterial in position, while the tentacular inturning is intermesenterial. 

 PI. XVI, fig. 117. represents the appearance of the invaginations at different levels, as met 

 with in a fully retracted polyp of Dichocmnia. The indentations occur about midway along 

 the radial extent of the disk, and vary greatly in extent, but in general diminish from above inwards. 

 The one to the left extends at this level over three mesenterial chambers, the entocoelic septum 

 being evidently notched, and thus permitting of the continuity of two really distinct tracts. The 

 middle imagination occupies only one exoccelic chamber, while the upper is still smaller, both in 

 width and radial extent. The discal ectoderm is exceptionally broad, being cut obliquely, and 

 the radial muscle libers are clearly seen. The tentacles themselves are also involved in the discal 

 invagination, but only as part of the disk, not as distinct organs. 



The depression of the discal wall results in the interruption of the mesenteries radially, so that 

 the latter are seen only toward their insertion in the polypal wall and in the stomodseal wall. 



On PI. XIII, tig. '.».">". taken from a retracted polyp of Fhvia fragum, a discal introversion 

 is seen neai- its termination, about midway along the transverse length of the mesentery: a few 

 sections below, the continuity of the mesentery is established. Such an appearance might easily 

 be mistaken for some tubular organ connected with a mesentery, but the phenomenon can be 

 readily explained as a result of the strong contraction of the mesenterial musculature. It is 

 easy to see how on full retraction of the polyp the discal wall will come to rest upon the septal 

 edges; then any further mesenterial contraction can draw the interseptal portion of the disk- 

 only downward, so that in transverse sections the latter appears as if actually inclosed within 

 the polypai cavity, along the same radius as the mesentery. 



As already mentioned, von Heider (1886) has described an imagination of the discal wall 

 in Astroides calycularis, but in this case the wall passes into the mesenterial chambers, without 

 in any way involving the mesenteries. It is evidently independent of the action of these 

 organs, and von Heider endeavors to explain the occurrence as dependent upon the interaction 

 of the tentacles and expulsion of the water during retraction of the polyp. 



Dr. O. Carlgren (1899), in his paper. "Giebt es Septaltrichter bei Anthozoen." discusses 

 Goette's view that the mesenterial funnels (Septaltrichter) found in the young of various 

 Actinian species are to be regarded as distinct organs. In the larvae of ]iini<i<l<s <j, fnmacea, Carl- 

 gren obtained appearances exactly similar to those figured by Goette, and shows conclusively that 

 they are merely contraction phenomena. Carlgren's figures compare most closely with figure 

 117. PI. XVI, and leave no doubt that the appearances are all due to the same cause, namely, 

 unequal contraction of different regions of the polyps during preservation. The occurrence of 

 tixed septa in corals renders it much easier to understand how the inequality is possible in this 

 group than in the case of the wholly soft-bodied anemones or their larva'. 



It may be conceived that the peculiar canal-like modifications, described by Fowler (18s7). 

 as occurring in certain mesenteries of Madrepora durvillei, have been produced by invaginations 

 during strong retraction of the polyp. There are however, some features in this case different 

 from conditions yet met with in corals, but on the other hand the modification seems altogether 

 at variance with our presenl knowledge of their morphology. 



