486 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



upper region of the apical polyps of Madrepora as below. In fig. 2, representing an apical polyp, 

 no radial openings occur on either side, though an uninterrupted continuity of the corallum for 

 such a distance appears to be somewhat unusual. The superficial canals in exsert corallites of 

 Madrepora are mostly longitudinal in direction; but transverse connections occur, and the canals 

 may be also interrupted by skeletal growths. Decalcified preparations show that the canal 

 system does not as a rule prolong the gastro-coelomic cavity at the aboral end of the polyps; in 

 both Pontes and Madrepora the polyps and their canals arc abruptly truncated. 



The internal canal system in the Porosa is morphologically basal in origin. For it is 

 established that the whole of the skeletotrophic tissue is derived from the primary basal disk. 

 and all its evaginations and invaginations are but so many foldings and complications of the 

 walls of this region of the polyp. The canals of the perforate corals may therefore be compared 

 with the basal communicating canals of colonial Actiniaria, such as those of PalytJwa among the 

 Zoanthea 1 . Though exerting a profound influence on the character of the corallum, the canal 

 system of the Porosa lias but little morphological significance, and in any natural classification 

 of corals appears altogether unworthy of the importance which has been assigned it by 

 systematists. 



The fundamental difference between colonial non-perforate and perforate corals may be 

 thus stated: In the Aporosa the gastro-coelomic cavities of the component polyps of a colony are 

 in communication only by superficial apertures along the common line of union of the column 

 wall and base, while the Porosa have in addition a means of communication by basal anastomosing 

 canals. 



Compared with that of the Actinian polyp the gastro-ccelomic cavity of the. Madreporarian 

 polyp is much more subdivided and intruded upon, both radially and peripherally, particularly 

 in its lower region. In addition to the mesenterial partitions, shared in common with the 

 Actiniaria, a like number of radiating septal invaginations usually occurs, while columellar, 

 spinous, and synapticular productions still further break it up centrally and peripherally. 

 Moreover, the calicinal wall itself is often produced upward as a peripheral, circular wall, 

 and, so far as it extends beyond the line of union of the column wall and base, divides the 

 ccelomic cavity, as well as its mesenterial partitions, into inner and outer moieties, the latter 

 constituting the cavity of the edge-zone or Randplatte. 



During expansion the upper part of living polyps is elevated for some distance wholly 

 beyond the corallum. and in distinct polyps the free portion is cylindrical, in form closely 

 recalling an Actinian polyp (figs. 16, -48). Here the subdivisions of the gastro-ccelomic cavity 

 are only mesenteric, and are arranged peripherally into entoccelic and exooelic chambers in a 

 strictly Actinian fashion. The fleshy parts of fissiparous genera likewise become extended 

 for several millimeters, the oral disk appearing as a meandering platform fringed by the zone of 

 tentacles, and the column wall on either side as a nearly vertical sinuous parapet: the cavity is 

 divided into entoccelic and exoccelic chambers, but not with any cyclic regularity. 



In retracted polyps not only is the oral region withdrawn within the calice, but the 

 perithecal wall becomes more nearly apposed to that immediately covering the skeleton, and 

 closely reproduces the outer corallar form, thus largely obliterating the intervening gastric space. 



Proximally the polypal cavity extends some distance within the corallum, or rather the 

 corallum has intruded within the polyp; and in these lower regions the cavity becomes 

 subdivided in a most complex manner by skeletal ingrowths. In addition, the skeletotrophic 

 endoderm becomes greatly thickened proximally, and further encroaches upon the chambers, 

 but apparently the mesenteries always cease before the lower termination of the polypal 

 cavity is reached." 



"Nothing like the peculiar obliteration of the polypal cavity which Sclater (lss(i) describes in Stephanotrochus 

 has been encountered. In all cases the ccelomic cavity persists as far as the proximal floor of the polyp. According 

 to I'.uiinir i ls'.i:;, p. LMVi), the polyp in Fungia docs nut desert the lower part of the calice, but remains adherent to 

 the basal plate. Yet even here Bourne finds that the primary and secondary mesenteries are carried upward as 

 growth proceeds, and are confined always to the upper moiety el' the calice; the lower moiety consists only of 

 chambers lined with endoderm and undivided by mesenteries. 



