446 JMEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 



only for about half a centimeter. Four of the.se, along with the eight complete mesenteries, 

 constitute the six pairs of protocnemes, while the remaining four repre.sent a pair of metacnemes 

 within each dorsal or suleular exoccrle. Notwithstanding this discovery, it is deemed convenient 

 to retain the term "' Ju//V(mh-/a-titiige'^ for the condition in which only the first four protocuemic 

 pairs are complete, whether other mesenteries are present or not. 



The fewness of the mesenteries in Por/'fe.s- and Madrfj/onc readily permits of their vertical 

 order of appearance and disappearance being followed in serial transverse sections, but the .same 

 can be carried out also in more complex, multicyclic forms. In species with a large number of 

 mesenteries the pairs are developed from above downward, as in Porifi/n and Madrepnra^ so 

 that the greatest number of cycles occurs in the more distal region of the polyps, and the mem- 

 bers of the last-formed cycles often traverse but a slight vertical extent, compared with that of 

 the oldest cycles. During the development of the mesenteries it is found (p. -i.o-l) that while 

 the first two or three pairs arise at or near the uppermost extremit}' of the polyp, that is, around 

 the oral aperture, the later pairs first appear some distance down the column wall, thence grow 

 in both directions, upward and downward. 



The mesenteries in adult corals as a rule terminate before the lower aboral extremity of the 

 polj'p is reached, hence this region is altogether unoccupied by any of the polypal tissues, except 

 those lining the skeleton (PI. IX, fig. ti7). The organs rarely occur below the upper half or two- 

 thirds of the vertical height of retracted polyps. Below, however, the septal invaginations 

 extend farther centrally than above, and the skeletotrophic endoderm becomes enormously 

 thickened, so that aboi'ally the ccclomic cavity is greatly diminished in extent. 



The restriction of the mesenteries in corals to the upper regions of the polyp should be 

 compared with the extent of their course in Actinians. Here the principal mesenteries usually 

 ti'aver.se the whole length of the column wall, and then extend across the basal disk toward the 

 center, where they often meet. On the other hand, it must be remembered that the aboral 

 extremity of the Madreporarian polyp does not altogethei- coincide with that of the Actiniarian 

 polyp. In the former, part of the basal disk is greatly invaginated. and its peripheral border, 

 where it passes into the coluum wall, is raised much aliove the actual extremity of the polyp. 



Perithecalh' the mesenteries maj' extend the full length of the column wall or edge-zone 

 {OrJjiceUa, iSV^/twasY/'a^rt, etc.), or may terminate in advance of it (Oculina). In the latter case 

 the attachment to the skeletotrophic tissue may be the first to cease, that on the column wall 

 remaining, or the columnar attachment may be the first to disappear. In Cladocora the 

 mesenteries rarely extend in a complete manner the whole length of the peripheral chambers, 

 the skeletotrophic attachment ceasing first (PI. VII, fig. 5-t). 



The mesenteries in Slderastrxa are characterized by a peculiar resorption of the peripheral 

 extremities, so that in the more central part of the polyp the organs extend much farther 

 vertically than in the peripheral region (PI. XXIII, fig. ISS). 



The polyps available are generally so small as not to permit of the mesenteries being 

 readily dissected out and viewed as a whole; but in serial transverse sections no interruptions 

 suggestive of (iK^'ntei'ial xtoinata have been encountered. Hence there is good reason to 

 suppose that mesenterial stomata are absent from Madreporarian polyps. The continual growth 

 upward of the polyp, and the resorption of the mesenteries below, characteristic of most 

 species, would in all probal)ility preclude the formation of such characteristic Actinian features. 



MESENTERIES IN GENERA REPRODUCING BY BUDDING. 



The adults of all other polyps here described are provided with a greater number of pairs 

 of mesenteries than the primary six of Pontes and Madrejjora. In the genera OrhiceUa 

 (p. 423), Solenastnea (PI. XI), Ocndhm, Slderastrxa (PI. XXIII), Cladocora (PI. VI), Astrangia 

 (PI. VI), and PhyUamj'm (p. 46-1), which reproduce asexually by budding, the mesenteries are 

 arranged in alternating hexameral cj'cles, and vary in size according to the cycle to which they 

 belong. The designation "cycle" is employed to include all the mesenteries having the same 

 radial extent, while the term "order" has reference to mesenteries which appeared at or about 

 the same time; the first has reference to their insertion on or distance from the stomodseum, and 



