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.
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.
In the above-given classification, the Scyphomedusae, formerly included with the Hydromedusae as Hydrozoa, are placed nearer the Anthozoa.
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.
As regards internal structure, polyps exhibit two well-marked types of organization, each characteristic Af one of the two classes, Hydrozoa and Anthozoa.
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.
The remaining family may, till further knowledge, be allowed to, cover four remarkable species, three of them resident on Anthozoa, one on an echinoderm.
(I) The polyp (hydropolyp) is of simple structure, typically much longer than broad, without ectodermal oesophagus or mesenteries, such as are seen in the anthopolyp (see article Anthozoa); the mouth is usually raised above the peristome on a short conical elevation or hypostome; the ectoderm is without cilia.
In the anthopolyp, on the other hand, the digestive cavity is always subdivided by so-called mesenteries, in-growths of the endoderm containing vertical lamellae of mesogloea (see Anthozoa).
CORAL, the hard skeletons of various marine organisms. It is chiefly carbonate of lime, and is secreted from sea-water and deposited in the tissues of Anthozoan polyps, the principal source of the coral-reefs of the world (see Anthozoa), of Hydroids (see Hydromedusae), less important in modern reef-building, but extremely abundant in Palaeozoic times, and of certain Algae.
(See ANTHOZOA, COELENTERA, CTENOPHORA and HYDROZOA.) (P. C. M.)
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