20 PEKCY SLADEN TRUST EXPEDITION 



Column "Wall (PI- 3, fig. 25 and PL 5, fig. 53). This term denotes the vertical portion 

 of body wall* outside the oral-disc. The greater part of its ectoderm [i.e. the part 

 lying against the corallum in the fully expanded condition) is the calicoblastic layer already 

 described. The mesoglaea between the calicoblastic layer and the inner endoderm is much 

 attenuated except at the attachments of the mesenteries to the corallite-walls (PL 2, 

 fig. 22). The inner endodermal layer is usually thin in the region above the enterostome 

 and contains a single layer of small more or less homogeneously stained nuclei. Towards 

 the base of the polyp the endoderm becomes, as a rule, highly vacuolated and consequently 

 appears much swollen, reticulated and transparent; nuclei are either absent, as in my polyps 

 of Galaxea, or, when present, arranged along its free margin. Algse here are rare or quite 

 absent. The mesoglsea is always extremely thin over the calicoblastic layer (PL 2, fig. 22). 



The column then bears a strong contrast to that of Actinians, in the presence of 

 the calicoblastic layer of ectoderm, the thinness of the mesoglsea and the absence of an 

 external or internal muscular layer. 



Edge-zone and Ccenosarc (PL 1, figs, l, 2, and PL 2, fig. 24). I use these terms in 

 the sense in which Professor Gardiner has defined them, viz., "The ccenosarc is that part 

 of the polyps in a colony which lies outside but not above (i.e. in expanded state) the thecse 

 of the several corallites. The ' Eand-platte ' of Heider and von Koch, and ' edge-zone ' of 

 Ogilvie, is then that part of the ccenosarc which lies over the free portions of the 

 corallites " (49, p. 361). It is to be noted that the distinction between the two structures 

 is a physiological not a morphological one. 



The edge-zone has an outer and an inner wall, the former being an extension of 

 the oral-disc, and the latter of the body-wall, and enclosed between those is an extension 

 of the gastro-vascular cavity of the polyp. All the mesenteries are extended into the 

 edge-zone. The structure of the outer wall of the edge-zone is more or less similar to that 

 of the oral-disc, and of the inner wall to that of the column-wall. The extent of the 

 edge-zone depends upon the degree of exsertness of the corallite ; in the case of Galaxea 

 fascicularis (Linn.), in which the corallites project conspicuously above the peritheca, 

 the edge-zone is very extensive as it covers the entire free surface of the corallite, whereas 

 in Goniastrea retiformis (Lam.), in which the corallites do not project above the general 

 surface of the colony, the edge-zone is absent. 



The ccenosarc covers the free surface of the peritheca, being only a continuation 

 of the edge-zone with which it is similar in structure. The extent of the ccenosarc 

 between neighbouring edge-zones depends upon the perithecal distance between the 

 corallites. In species like Goniastrea retiformis (Lam.), in which the corallites are 

 so closely aggregated that there is no peritheca between them, the ccenosarc is also 

 absent. 



Tentacles (PL 1, figs. 3, ll, 12, PL 2, fig. 19, PL 3, figs. 26, 28 and PL 5, fig. 44). 

 The tentacles arise from the oral-disc as hollow vertical outpushings of the inter-mesenteric 



* Body-wall comprises the entire wall of the polyp surrounding the gastro-vascular cavity, viz. the oral- 

 disc with the tentacles above, the basal-disc below, and laterally between these the column-wall. The basal-disc 

 is pushed up by the columella and paliform lobes, its ectoderm being the continuation of the calicoblastic layer 

 of the column-wall and its endoderm usually vacuolated as in the lower part of the column-wall. 



