floors stretch completely across the
body, uninterrupted by vertical partitions, which, if they exist at all,
pass only from floor to floor, instead of extending unbroken through
the whole height of the body, as in all Polyps. Where, on the contrary,
transverse floors exist in true Polyps, they never cut the vertical
partitions in their length, but simply connect their walls, stretching
wholly or partially from wall to wall.

In the Astraeans, the multiplication of tentacles is more definite and
limited, rising sometimes to ninety and more, though often limited to
forty-eight in number, and the transverse floors between the vertical
partitions are more complete than in the Fungidae. The Porites have
twelve tentacles only, never more and never less; 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, showing a kind of
subordination of the whole community to these larger individuals, and
thus sustaining the view expressed above, that the combination of many
individuals into a connected community is among Polyps a character of
superiority when contrasted with the isolation of the Actiniae;. In the
Sea-Fans, the Halcyonoids, 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 complicated in structure than those of the preceding Polyps.

According to the relative complication of their structure, these animals
are classified in the following order:--

STRUCTURAL SERIES.

Halcyonoids: eight tentacles in pairs, lobed around the margin; always
combined in large communities, some of which are free and movable like
single animals.

Madrepores: twelve tentacles, alternating in six larger and six smaller
ones; frequently a larger top animal standing prominent in the whole
community, or on the summit of its branches.

Porites: twelve tentacles, not alternating in size; system of connected
beams.