MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 411 



In fissiparous genera like Favia and 7sophyllia, in which one or a few oral apertures may 

 occur mi a single disk, a single .wall is common to each disk, I ml is separable from those adjacent, 

 a- in the cases just described. The fissiparous conditions met with in these genera become 

 more complex in Mdnicina, Mseandrina, Pectinia, and Colpophyllia. Here the column wall, 

 like the disk and the tentacular zone, is common to a large number of oral apertures, hut 

 along the thecal ridges (collines) a longitudinal groove occurs, separating the column of two 

 adjacent systems. A further condition occurs in Agaricia. New polyps seem to arise by fission, 

 and each possesses its own system of tentacles; there is. however, no precise boundary line or 

 gl'OOve between the column wall of adjacent polyps. A prominent thecal ridge imperfectly 

 mark- oil One poly}) from another (PI. NXIY. fig. L62), hut no external indication is afforded 

 that the column wall becomes adherent to the coralium along its apex. 



Madrepwa is another genus in which no external demarcation occurs between the superficial 

 tissues of the various polyps making up a colony; it is impossible to say where the column wall 

 of one polyp ends, and that of another begins. As shown on PI. I. fig. 1./. representing a fully 

 expanded apical polyp, the free cylindrical region, which should undoubtedly lie regarded a- a 

 column, passes directly into the superficial covering of the colony: hut on this there is no groove 

 limiting the column of one polyp from those surrounding it. 



In simple corals, and around the periphery of colonies, the lower or proximal extremity of 

 tin' column wall is closely adherent to the coralium. and upon decalcification its uninterrupted 

 passage into the basal skeletotrophic tissues can be followed, the histological structure of the two 

 differing greatly. The upper distal margin of the column continues to grow- upward, the lower 

 extremity keeping pace with it. and the skeleton below is thus left exposed. Usually foreign 

 growths, particularly Nullipores, in time settle upon the exposed part of the coralium; or it may 

 be attacked by destructive agents,' such as boring sponges or mollusks, or by tubiculous worms. 



At the actual boundary of the column wall and basal disk a thin deposit of calcareous matter 

 usually takes place, which in coral terminology is known as the "epitheca." This generally 

 shows sie-ns of stratification or wrinkling, the thickened lines representing periods when the 

 upward growth of the polypal margin was not proceeding rapidly, and consequently more 

 calcareous formation took place. In the early stages of Manicina areolata, the column wall 

 practically envelops the whole of the coralium. and all stages in its growth upward, according 

 as the colony enlarges, can be obtained. In the skeleton the epitheca is (dearly seen as a thin 

 calcareous layer resting upon the edges of the costse, its upper margin indicating both the 

 proximal extremity of the column wall and the commencement of the skeletotrophic tissue- 

 when the colony wa- alive. The region at which the epitheca is formed is (dearly seen on PI. XIX. 

 fig. 137. representing a section through a young polyp of Manicina, and also on PI. XIV. 



RANDPLATTE OB EDGE-ZONE, CCENOSAEC, CCENENCHYME. 



The term "Randplatte" was originated by von Heider(1881, p. 4). when describing the exter- 

 nal features of the Mediterranean Cladocora, to include the continuation beyond the crown of 

 tentacles of the soft parts of the polyp over the border of the calice. It has since been exten- 

 sively employed in Madreporarian literature by Fowler. Bourne, and Miss Ogilvie, the latter of 

 whom introduced "Edge-zone" as its English equivalent (1896, p. L08). Referring to the name, 

 G. von Koch (1886, p. 342), in a foot-note, draws attention to the fact that the region alluded to 

 is no structure "sui generis." and therefore possesses no independent morphological significance. 



In expanded coral polyps there is really no demand for such a descriptive term, as in this 

 state the column wall stretches vertically, in undivided continuity, from the margin of the tentacu- 

 lar crown to its line of union with the wall of the surrounding polyps, and. except for a stronger 

 development of the endodermal musculature above, the histological structure of the wall is the 

 same throughout. Most of the mesenteries also extend the whole length of the column. Where, 



" In Astranffia solitaria the incrusting Nullipores sometimes grow upward with such rapidity as to cover the whole 

 of tin- external surface of the corallites, displacing the pericalicular part of the polyp. They may even extend over 

 the thecal edge so as to sensibly diminish the aperture through which the polyp protrudes. 



