CH. VI MABINE FAUNA OF TALIS SE SHOBES 121 



or tliecse {th) which give support to the coral polypes. Each 

 theca is supported by a number of vertical plates project- 

 ing centripetally from the inner side of the cup. These are 

 called the septa (S). They serve as buttresses to give 

 strength to the theca. All this skeleton is composed of 

 nearly pure carbonate of lime (a salt present in solution in 

 all sea-water), which is absorbed from the water by the 

 coral polypes and secreted again by them as the solid 

 skeleton. 



The soft or fleshy parts of the coral completely cover 

 the skeleton, following all the ridges, septa and other ir- 

 regularities, so that if we could imagine the soft parts 

 drawn or pulled off from the hard, they would bear an 

 exact impression on their under side of the surface of the 

 skeleton. On the outside of the cups and on the connect- 

 ing basal plate the soft parts are composed of a simple 

 flat plate of fleshy substance (f-l.). Within the cups are 

 situated the polypes, which are directly continuous with 

 the fleshy lamina covering the rest of the coral. The main 

 points in the anatomy of a coral polype are soon described. 

 On the upper disc there is a crown of tentacles (t t) sur- 

 rounding a slit-shaped mouth (M), as in a sea-anemone. 

 The mouth leads into a short flattened throat (St) tech- 

 nically known as the stomodseum, and this opens below 

 into the general cavity [G c) of the polype. The general 

 cavity of the polype is divided into a number of chambers 

 or compartments by the septa and mesenteries, the arrange- 

 ment of which may best be studied in the diagrammatic 

 transverse section, fig. 14. The upper half of the dia- 

 gram represents a section of the theca in the region of 

 the dotted line x x' . The mesenteries iyn in) are here seen 

 to be thin vertical plates of flesh connecting the walls 

 of the theca with the throat; the septa {SS), composed 

 of vertical plates of lime covered with a thki layer of 



