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, 



Fig. 47.— Diagram to illustrate anatomy of Galaxea esperi. 

 1. The theca. 7. Mesentery. 



After Hickson. 



2. The septa. 



3. The basal plate. 



4. Flesh covering the theca. 



5. Tentacles. 



6. Mouth. 



8. Lower edge of stomodaeum. 



9. Free edges of mesenteries beneath 



the stomodaeum. 

 10. The mesenteric filaments. 



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 

 coelenteron, 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 



