AGE OF CORAL REEFS. 195 



and in them the whole solid frame presents a 

 complicated system of connected beams. The 

 Madrepores have also twelve tentacles, but they 

 have a more definite character than those of the 

 Po rites, on account of their regular alternation 

 in six smaller and six larger ones ; in these also 

 the transverse floors are perfect, but exceedingly 

 delicate. Another remarkable feature among the 

 Madrepores consists in the prominence of one of 

 the Polyps on the summit of the branches, show- 

 ing a kind of subordination of the whole com- 

 munity to these larger individuals, and thus sus- 

 taining the view expressed above, that the com- 

 bination of many individuals into a connected 

 community is among the Polyps a character of 

 superiority when contrasted with the isolation 

 of the Actiniae. In the Sea-Fans, the Halcyo- 

 noids, as they are called in our classification, 

 the number of tentacles is always eight, four 

 of which are already present at the time of 

 their birth, arranged in pairs, while the other 

 four are added later. Their tentacles are lobed 

 all around the margin, and are much more com- 

 plicated in structure than those of the preceding 

 Polyps. 



According to the relative complication of their 

 structure, these animals are classified in the 

 following order : — 



