'39 



may be supposed to be going on especially activel}^ the layer 

 is more hyaline and where not granular. The distances 

 between the thickenings of the ectoderm on the septal edges 

 correspond more or less to the distances between the ridges 

 on the sides of the septa, and seem to lie over their ends. 

 They would hence fall on the so called " centres of calcifica- 

 tion." When I first examined microscopically the skeletons 

 of corals, I thought that these centres corresponded to the 

 tubes of boring organisms, which became densely packed 

 with the dust caused by grinding. Such organisms do tend 

 in colonial reef-corals to bore along these centres, indicating 

 perhaps that they are lines of least resistance. jNIay not these 

 centres be directly due to the thickenings of the calicoblastic 

 layer ? I can only regard the layer as an enormous syn- 

 citium, and for the growth of a septum there would seem to 

 be a flowing up of the protoplasm on either side. Where the 

 two layers of protoplasm fuse, i.e., immediately over the 

 " dark line " joining the " centres of calcification " there is an 

 extensive formation of corallum. This takes on the crystal- 

 line form, but the regular arrangement is not seen until after 

 the formation of the " growth lamellae " of the septal sides. 

 The " centres of calcification " would, on this view, represent 

 aggregations of crystals of carbonate of lime not arranged in 

 any determinate direction. The radiations from these would 

 then represent lines of irregularly arranged crystallisation. 



General Ectoderm figs. 4-8 . — The ectoderm is every- 

 where extremely well preserved and shows its structure 

 admirably. It varies in different polyps only in accordance 

 with their state of contraction, outside the tentacles often 

 appearing as if knobbed 'fig. 4). Cell outlines cannot usually 

 be distinguished, but it is an epithelium of a narrow, 

 elongated, columnar facies with a broad, crowded layer of rod, 

 or oval-shaped, densely granular nuclei. The latter vary 

 considerably with the amount of vacuolation and the presence 

 or absence of gland cells, but are for the most part found in 

 the outer half of the epithelium. The outer or free edge 

 presents an appearance of longitudinal striation, so that it is 

 probably in life ciliated all over. Over the structureless 

 lamella the protoplasm forms a finely granular network, in or 

 above which a few rounder nuclei may be distinguished. These 

 belong to irregularly-shaped sense cells, some of which are 

 represented in the figures. 



Gland cells occur of two kinds, mucous and granular, and 

 can be best distinguished in tissues stained with thionin and 

 orange green. The mucous cells stain deep blue, and the 

 granular in accordance with their ripeness from yellow to 

 black. The mucous cells are of the typical goblet-shape, and 

 are situated in the outer half of the layer. Most of the 



