412 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



however, the calicinal wall extends peripherally far upward within the cavity of the polyp, then 

 upon retraction of the latter the upper region of the column becomes drawn within the calice, hut 

 the lower region, still with the mesenteries attached to it, remains outside. It is to this external 

 area of the column wall, often sharply marked off in retracted polyps, that the term "•edge-zone" 

 is usually restricted. As a result of the same upgrowth of the calicinal wall, the ccelenteron 

 likewise becomes separated into calicinal and pericalicinal or perithecal portions, each parti- 

 tioned into chambers by the mesenteries, and less so by the septal and costs (PI. VII, fig. 54). 



Among corals like Por/'f, * and Sid nt-xt/wi, in which the calicinal wall is common to adjacent 

 polyps, and the septa are but little or not at all exsert, there can possibly be no extrathecal, or 

 rather pericalicinal or perithecal, continuations of the tissues, and no edge-zone. 



By "edge-zone" Dr. Ogilvie (p. 108) understands " that the mesenteries of the interseptal 

 loculi are continued into the intercostal loculi," thus giving a more precise meaning to the term 

 than was done by von Heider. Among all the forms here studied, which are provided with a 

 perithecal continuation of the gastric cavity into intercostal loculi, Madrepora is the only one in 

 which the mesenteries also are not prolonged perithecally. In this genus the superficial covering 

 of the colony is continuous with the column wall of the polyp, and, as shown on PI. I. tig 2, the 

 ccelenteron is directly continuous over the edge of the theca with the superficial canals, but there 

 is never any trace of external mesenteries. In the expanded polyp the mesenteries are seen to 

 pass from the extruded column wall directly into the calice. and the column wall below, unsup- 

 ported by mesenteries, rests directly upon the skeletal echinulations. 



The precise definition given to the edge-zone affords Miss Ogilvie the opportunity of accom- 

 plishing the same for the somewhat loosely employed term "Ccenosarc." By this the authoress 

 (p. 108), following Bourne (1888, p. 26), signifies "an extrathecal part into which the mesenteries 

 do not continue." Ccenosarc will, of course, consist of two distinct tissues: the skeletal covering 

 proper (base), and the superficial covering to the colony (column wall), the two separated 

 more or less by a continuation of the gastro-ccelomic cavity." By universal acceptation, " Coen- 

 enchyme" is the calcareous deposit originating from the ccenosarc, and this is only laid down by 

 the skeletotrophic layer, the inner of the two external tissues. According to the definition of 

 ccenosarc and ccenenchyme just given, Madrepora alone, among all the forms available for study, 

 is characterized by these structures: that is to say. the only genus in which the perithecal walls of 

 the polyp are without mesenteries (PI. I). 



One of the most illustrative examples in this connection is Oculina. In all the definitions of 

 the. genus one of the characteristics given is the. presence of a solid ccenenchyme. Yet through- 

 out young colonies, and in the growing regions of others, the mesenteries are prolonged 

 perithecally, so as to extend as far as the spiral groove of separation of the superficial tissues of 

 the different polyps, and the corresponding grooves on the skeleton are determinable throughout. 

 It is only in the older regions of large colonies that the mesenteries do not extend the whole 

 length of the column wall, and the skeletal surface then becomes perfectly smooth, with an 

 absence of grooves or costal ridges. Under such circumstances it become-, impossible to draw 

 any sharp line between edge-zone or column wall and ccenosarc. The latter is merely the extra- 

 capsular region of the polyp into which the mesenteries are not prolonged. 



Bourne (p. ■2(<) states that "a common ccenosarc is due to nothing more than a persistent 

 connection between the ' Kandplatten ' of adjacent polyps, and that the two structures are 

 homologous." This undoubtedly holds for some forms, e. g. . Galaxt a, but the first portion of 

 the definition can scarcely be regarded as applicable to cases like Madrepora, where, by deliui- 



"At the points where the ccenosarc rests upon the costal ridges or echinulations the two coverings are combined, 

 and the skeleton is Inn- overlaid only by the superficial ectoderm, the niesoidiea, ami the calicohlastic ectoderm 

 (Pis. I, II). The perithecal gastro-ccelomic cavity then becomes represented by canals, often reticular in char- 

 acter. Fowler I isss, p. 7, PI. XXXII, figs. '_', :: i shows that in Amphihelia ramea the direct adherence of the polypal 

 wall to the ski 'let on may become very broad, the canals being, as it were, pushed apart from one another and greatly 

 narrowed. 



iif the canals in Comopsammia Gardiner (1900, p. 361) observes: "The ccenosarca) canals in fact are simply 

 extrathecal portions of the ccelentera of the different polyps, which serve to connect their intrathecal or gastrovascular 

 port i' his." 



