COELENTERATA 69 
The skeleton commences to appear by the ectoderm of the 
base secreting a flat plate between it and the substance on which 
the young actinozoan is fixed. From this plate a number of 
radially-arranged vertical ridges grow up (Fig. 47, 2). These are, 
Fic. 47.—Diagram to illustrate anatomy of Galaxea espert. After Hickson. 
1. The theca. 7. Mesentery. 
2. The septa. 8. Lower edge of stomodaeum. 
3. The basal plate. 9. Free edges of mesenteries beneath 
4. Flesh covering the theca. the stomodaeum. 
5. Tentacles. 10. The mesenteric filaments. 
6. Mouth. 
like the rest of the skeleton, secreted by ectoderm ; and as they 
grow they push the ectoderm and the skin of the base up into 
the coelenteron. These vertical plates are termed septa. 
Partly by the fusion of the external edges of the septa, and 
partly by the upgrowth of a circular rim from the basal plate, 
a circular ridge is formed. This ridge forms the wall of 
the cup or theca, and like the septa it projects into the 
eoelenteron, pushing the body-wall before it; occasionally a 
second circular rim, external to but concentric with and close to 
the first, is formed, the epitheca, and this forms a ridge round 
