414 MEMOIRS OF THE xVATIONAL ACADEMY OF SCIENCES. 



in a form like hoplnjllia, where the septal and costal spines are very variable in the amount of 

 development. The external g-rooves which separate the verrucal ridges correspond with the line 

 of attachment of the internal mesenteries, and are always smooth. 



Apparently there are no permanent apertures in the column wall of Madreporarian polyps, 

 such as zoophytologists are familiar with in the '" Cinclides " of the Sagartids among the Actiniaria. 

 Thi-ough these latter the thread-like ^'Acontia," loaded with nematocysts, are extruded when the 

 polyp is irritated. The majority of coral polyps, however, have the power of extruding 

 prolongations of the mesenteries hearing coiled mesenterial filaments along their edge (p. 47.t), hut 

 these can evidently perforate any portion of the superficial tissues, the disk equally with the 

 column wall. Careful examination of the body wall, before the filaments are extruded, fails to 

 reveal any apertures, and their irregular distribution, sometimes over nearly the whole external 

 surface of the polyp, would suggest that the apertures are merely temporary and may be 

 produced at any point. On PI. VIII, tig. 64, is represented a section through a portion of the 

 column of a polyp of Orhlcella amudariji through which the filamental part of a mesentery is 

 extruded. No histological modification whatever can be made out in the wall itself; the aperture 

 is a mere interruption of the layers for the passage of the mesentery and its filament. Upon the 

 polyps settling down after irritation the filaments are slowly indrawn, and ultimately no external 

 indications remain of the apertures through which they protruded. In some cases the openings 

 have been observed to remain distinct for a short time after the indrawal was completed, ))ut the 

 injury, if such it can be regarded, was soon completely healed. 



On full expansion of the polyp the column may extend for some distance above the coral- 

 lum, and is either cylindrical, oval, or irregular in form. Proximally, where it is fixed to the 

 skeleton, it assumes the outline of the individual corallites, and hence maj' be circular, polygonal, 

 or irregulai'. In species of Sidermtr^'d and Agarivia the column appears never to be raised luuch 

 above the general surface of thecorallum, and in forms \\]s.q 2I»:'andrin(i, with incomplete j^olypal 

 separation, the colunni on both sides rises for man}' millimeters as a vertical expansion, with a 

 deep valley separating one polypal row from another. 



The form and position assumed l)v the intercalicular portion of the colunm wall upon retrac- 

 tion of the poU'ps varies greatly. In most cases the upper region of the column becomes folded 

 inwardly over the edge of the theca, while in some it is mej'ely drawn downward. In the 

 former condition it either comes to lie inclined downward against the ol)li(jue septa {Jfa/u'ciiia, 

 Ma^diidrina, etc.), or, by the action of the endodermal circular muscle, it extends horizontally, 

 terminating in a circular margin which nearly' meets at the center, and thus almost covers 

 the disk below (PI. X. tig. 74). In Mndnqvira the wail becomes merely drawn within the calice 

 without any overfolding (PI. I, fig. 2); in Slderastrira and Agaricla the column and disk 

 are simply depressed, and come to rest upon the skeleton, leaving the tentacles and mouth wholly 

 exposed (PI. XXII, fig. 150). 



Variations in the position assumed by the column wall on retraction of the polyps are some- 

 times observable even in the same species. Thus the wall in Porites chtrdvia may be slightly 

 folded over the disk, or, as in Sidcrantrim, it may merely come to rest upon the corallar surface, 

 the tentacles and disk remaining exposed (PI. IV, figs. 34 and 35). 



Among the skeletonless Acfinnc the coluum wall is usually of some thickness, so as to 

 give more or less rigidity to the body of polyp, but in the Madreporaria, where support is 

 afiorded by the skeleton, the polypal wall is nearly always a thin, delicate, often transparent 

 structure. In both groups the thickness of the wall is maitdy determined by that of the middle 

 layer — the mesogkea, as both the ectodermal and endodermal epithelia vary comparatively little. 

 By contrast with that of most anemones the mesoglcea in the column wall of corals is, as a rule, 

 little more than a mere separating lamella Ijetween the inner and outer layers, except along the 

 line of attachment of the mesenteries, where it becomes somewhat thickened in a triangular 

 manner. 



The thickness of the colunm wall is also partly dependent upon the state of expansion or 

 retraction of the polyp. On full distention all three layers Ijecome greatly attenuated, the 

 ectodermal and endodermal cells largely diminished in height, and the mesoglcea scarcely distin- 



