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 
Porites, 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 Actiniz. 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 : — 
