410 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



the retracted condition it is not .so evidcMit. :ind is further eonfiised because one portion of the 

 coliunn may be within the calice and another outside: also, owing to the colonial habit of most 

 sjiecies, the line of separation of tiie column wall of one polj'p from another is not always readily 

 determinable. 



The r(>t(>ntion of the word column liecomes absolutely- necessar}- for a correct appreciation 

 of the morphological relationships of the corresponding region.s in the various types of coral 

 growth. The region admits of a very precise definition, and, except in a few instances, of distinct 

 limitations on the living or preserved colony. In the Actiniaria the column includes the whole of 

 the polj-pal wall between the basal disk and the oral disk, the latter limited peripherally by the 

 outermost c^'cle of tentacles. It is also usually distinguishaljle from the rest of the polyp In- 

 structural differences, especially in the stronger development of the nmsculature distally. 



Embryological results indicate, as was first established by Professor \on Koch (1<S82), that 

 in Madreporarian corals the basal disk of the larva or young polyp first gives rise to the skeleton, 

 and. however complicated the lattci' ultimately ))ecomes. the tissues lining it directly (skeleto- 

 trophic or skeletogenic) are morphologically those of the base. It follows from this that the line 

 at which the skeleton-pi'oducing tissues pass into the superficial tissues is the boundarv between 

 the true basal disk and column (PI. XIX. fig. 1.37). The latter will thus include all the superficial 

 part of the polypal wall between this boundarv and the outermost row of tentacles, and nowhere 

 takes any part in the formation of the skeleton. Colunui wall and oral disk will thus practically 

 correspond with "oral body wall," and basal disk with "aboral l)ody wall." as these terms are 

 employed by Fowler, Ogilvie (ISIK), p. 107), and others. By bodj- wall or polypal wall I under- 

 stand the whole or any part of the wall of the polyp — base, column, and oral disk. 



In simple polyps, and at the margin of colonial polyps, the boundary between the basal wall 

 and the column wall is entire, and is indicated l>y a marked histological difference; but in colonial 

 polyps, elsewhere than at the margin, interruptions exist which permit of free communication 

 between the internal cavities of the various polyps constituting a colony (PI. XII, fig. 87). Mesen- 

 teries are attached for some distance along the basal skoletotrophic wall, and then pass up the 

 colunui wall, and in the cas(> of the complete members are contiiuied across the disk and down 

 the stomodieum. 



The column wall, as above defined, is easily distinguished in the simple polyps of Astrangia, 

 and Phyllangid. (PI. V, fig. 4t>): ))ut in colonies, where the asexually produced polyps remain 

 comiect(>d with one another, the limitation of the wall of the individual polyp is not always 

 readily determina))le externally. Many colonial genera, including such as (h'liiceJla, Siderastrira, 

 and Piiritix, display a smootii ])()lygonal groove which represents the external line of demarcation 

 of the polyps. The sup(M'ficial tissu(>s are in partial continuity with the skeletotrophic tissues 

 along these grooves, either directly or through the intermediation of the mesenteries, and the 

 groove is therefore incapaV)lc of elevation above the skeleton, even on full expansion (PI. IX. 

 fig. G7). 



The two or three polyps, wliich as a rule constitute the sub-<-olonies of Cladocora, afl'ord 

 interesting .stages in the separation of the body wall of polj-ps primarily united (PI. VI, lig. 48). 

 Usually each ])olyp presents a free portion along its lower margin, where the ectoderm of the 

 column can be seen to pass into that of the base, while the remainder is united with the termina- 

 tion of the wall of the other polyps, the line of union being indicated by a groove. As the 

 polyps increase in size this line of connection diminishes in extent, the communication between 

 the cavity of one polyp and of the other ceases, and ultimately the polyps separate, though 

 usually not before each has given rise to one or more buds. 



The polyps of Ocid'ina (PI. XXII. fig. 1-1:9) are .spirally arranged, and as a rule widely 

 separated; in the older regions of colonies the limitations of the individual (•olunins are not 

 readily seen, but can be easily made out in young colonies, and at the growing regions of others. 

 Where the boundary is indicated the pericalicular mesenteries extend as far as the limitations 

 of each polyp; but where the polyps have become widely separated, the mesenterial pi-olongations 

 cease before the limitations of the polyps are reached, and then no actual boundarj- between 

 one and another persists. 



