With the single exception of the Mollusca, no division of the animal kingdom contributes such important and numerous indications of its past existence as the Actinozoa.

In the Palaeozoic rocks the majority of corals belong to the division Rugosa, these seeming to have filled the place now taken by the sclerodermic Zoantharia. Until quite recently it was believed that all the Rugosa were Palaeozoic, with the exception of the genus Holocystis, represented in the Cretaceous period (Lower Greensand) by the single species H. elegans. Recent researches, however, have brought to light the existence in our present seas of at least two genera (Haplophyllia and Guynia), which belong-to the Rugose family of the Cyathaxonidae; and certain Tertiary Rugose Corals (Conosmilia) have also been described (Martin Duncan). Of the families of the Rugosa, the Cyathophyllidae and Cystiphyllidae are exclusively Palaeozoic ; the Cyathaxonidae are Palaeozoic, but are represented by two living genera; and the Stauridae are represented in the Silurian rocks (Stauria), Devonian (Metriophyllum), Permian (Polyacelia), and in Tertiary deposits (Conosmilia).

The Zoantharia sclerodermata, though attaining their maximum at the present day, nevertheless are well represented in past time, beginning in the Silurian period. The Perforata are principally represented by the Favosilidae during Palaeozoic time, though other forms of this section are not unknown ; but, like the Aporosa, they attain a much greater development in the Mesozoic and Kainozoic deposits.

The Zoantharia sclerobasica are hardly known as fossils, but the Miocene deposits of Piedmont (Middle Tertiary) have yielded a species of Antipathes.

The Zoantharia malacodermata, from the soft nature of their bodies, are obviously incapable of leaving any traces of their existence ; though we are by no means therefore justified in asserting that they did not exist in past geological epochs.

With the reference of the Helioporidae to the Alcyonaria, the range of this order has been enormously increased in past time. Formerly no example of the order was known as occurring in any Palaeozoic stratum, the so-called Protovirgularia of the Lower Silurian being probably a Graptolite. Now we know of various abundantly distributed Alcyonarian corals in the Palaeozoic rocks, the most important being the Heliolites of the Silurian and Devonian. The allied genus Polytremacis is Cretaceous. Of the Gorgonidae two genera (Mopsea and Websteria) are found in the Eocene ; and the genus Corallium, doubtfully quoted from the Jurassic and Cretaceous, is found in the Miocene, which has likewise yielded examples of Isis, Gorgonia, etc. The Pennatulidae commence in the Eocene, with Graph-tdaria. The Tubiporidae (unless the Palaeozoic Syringoporidae be referred here) are unknown as fossils, and the Alcyonidae are not known to occur till the Pliocene is reached.

The Ctenophora, being wholly destitute of hard structures, are not known at all as occurring in the fossil condition.