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.
Astraeans: tentacles not definitely limited in number, though usually not exceeding one hundred, and generally much below this number; transverse floors. Maeandrines, generally referred to Astraeans, are higher than the true Astraeans, on account of their compound Polyps.
Fungidae: indefinite multiplication of tentacles; imperfect transverse beams.
Actiniae: indefinite multiplication of tentacles; soft bodies and no transverse beams.
If, now, we compare this structural gradation among Polyps with their geological succession, we shall find that they correspond exactly. The following table gives the geological order in which they have been introduced upon the surface of the earth.
GEOLOGICAL SUCCESSION.
Present, Halcyonoids.
Pliocene, \
Miocene, } Madrepores.
Eocene, /
Cretaceous, \ Porites
Jurassic, } and
Triassic, } Astraeans.
Permian, /
Carboniferous, \
Devonian, } Fungidae
Silurian, /
With regard to the geological position of the Actiniae we can say nothing, because, if their soft, gelatinous bodies have left any impressions in the rocks, none such have ever been found; but their absence is no proof that they did not exist, since it is exceedingly improbable that animals destitute of any hard parts could be preserved.