If a polyp, such as Hydra, be regarded simply as a sessile actinula, we must certainly consider the polyp to be the older type, and it may be pointed out that in the Anthozoa only polyp-individuals occur.
In 18J5, however, Owen included under Lamarck's term Radiaria the Echinodermata, Anthozoa, Acalepha and Hydrozoa, while Agassiz also clung to the term Radiata as including Echinodermata, Acalepha and Polypi, regarding their separation into Coelenterata and Echinodermata as "an exaggeration of their anatomical differences" (Essay on Classification, London, 1859).
Orders: Ciliati (Rotifera), Denudati (Hydroids), Vaginati (Anthozoa and Polyzoa), Natantes (Crinoids).
Classes: Echinodermata, Bryozoa, Anthozoa, Acalephae, Hydrozoa.
Orders: Anthozoa and Cylicozoa.
The Hydrozoa are thus shown to belong to the group of Coelenterata Cnidaria, and it remains to consider more fully their distinctive features, and in particular those which mark them off from the other main division of the Cnidaria, the Anthozoa, comprising the corals and sea-anemones.
The Anthozoa differ from the Scyphomedusae in having no medusoid form; they all more or less resemble a sea-anemone, and may be termed actinioid.
At the parting of the ways which led, on the one hand, to modern Scyphomedusae, on the other to Anthozoa (III.), it is probable that the common ancestor was marked by incipient mesenteries and by the limitation of the sexual cells to endoderm.
To Anthozoa - this group abandoned its power of adult locomotion by swimming.
- Sensory to that of the Anthozoa, but this has been dis cells from the retina proved by the most recent investigations of o f Char y b d a e a, Hein (4) and Friedemann (3), who have shown highly magnified.
By some authorities the Scyphomedusae have been removed from the Hydrozoa and united with the Anthozoa in a common group termed Scyphozoa.
There remains only the third feature, the endodermal gonads, as an argument for uniting the Scyphomedusae with the Anthozoa, against which must be set all the peculiarities of medusan organization in which the Scyphomedusae resemble the Hydromedusae.
In the subdivision Anthozoa, comprising the sea-anemones and corals, the individual is always a polyp; in the Hydrozoa, however, the individual may be either a polyp or a medusa.
For further details of colonyformation the reader is referred to the articles Anthozoa and Hydromedusae.
The hard calcareous substance to which the name coral is applied is the supporting skeleton of certain members of the Anthozoa, one of the classes of the phylum Coelentera.
In Actinia and its allies, and most generally, though not invariably, in Anthozoa,the stomodaeum is not circular, but is compressed from side to side so as to be oval or slit-like in transverse section.
Hertwig.) The Anthozoa are divis ible into two sub-classes, sharply marked off from one another by definite anatomical characters.
C. Bourne, "Anthozoa," in E.