MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 411 



In tissiparous genora like Favla and 7s"j//ii//h'(i, in wliit-li one or a few oral apertui-cs may 

 occur on a single disk, a single wall is common to each disk, but is separable from those adjacent, 

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

 more complex in Maiu'cinxu Mieandrlna , PecUxlti, and ColpophyUia. Here the column '\\'all, 

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

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

 adjacent systems. A further condition occurs in Agar'wla. New polj'ps seem to arise bj' fission, 

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

 groove between the column wall of adjacent polyps. A prominent thecal ridge imperfectly 

 marks oft' one polyp from another (PL XXIV, lig. 162), but no external indication is aflorded 

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



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

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

 of one polyp ends, and that of another begins. As shown on Pi. I, fig. Xa, representing a fully 

 expanded apical pol^'p, the free cjdindrical region, which should undoubtedly be regarded as a 

 column, passes directly into the superficial covering of the colonj-; but 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 

 the column wall is closely adherent to the corallum, and upon decalcification its uninterrupted 

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

 diftering 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 Nulliport's," in time settle upon the exposed part of the C(jrallum; or it may 

 be attacked by destructive agents, such as boring sponges or mollusks, or l)y tubiculous worms. 



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

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

 shows signs of stratification or wrinkling, the thickened lines representing periods when the 

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

 calcareous formation took place. In the early stages of Manieina areolafa. the column wall 

 practically envelops the whoU; of the corallum, and all stages in its growth upward, according 

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

 calcareous layer resting upon the edges of the costs, its upper margin indicating T)oth the 

 proximal extremity of the columij wall and the commencement of the skeletotrophic tissues 

 when the colony was alive. The region at which the epitheca is formed is clearly seen on PI. XIX, 

 fig. 137, representing a section through a j'oung polyp of ILtnieina^ and also on PI. XIV. 



EANDPLATTE OR EDGE-ZONE, CCENOSARC, CCENENCHYME. 



The term " Randplatte" was originated by von Heider (ISSI, p. 4), when d(iscribing the exter- 

 nal features of the Mediterranean Vlaclocora^ 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 emploj^ed in Madreporarian literature by Fowler, Bourne, and Miss Ogilvie, the latter of 

 whom introduced "Edge-zone" as its English equivalent (1S96, p. lOS). Referring to the name, 

 G. von Koch (ISlSfi, p. 842), 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 contiiniity, 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 endoilermal nmsculature al)ove, the histological structui'e of the wall is the 

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



"■ In Astrangia solitaria the incrusting Nullipores sometimes grow upward with such ra])idity as to cover the whole 

 of the 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. 



