CORALS
CORALS (Coralliaria). Under the general name of
Vj " Coral-Animals " are included all those members of
the Actinozoa which have the power of secreting hard struc
tures of the nature of a skeleton. Whether this skeleton be
continuous or discontinuous, of conspicuous dimensions or
simply of microscopic spicula, it constitutes what is known
properly as the " coral " or " corallum ; " and the animals
which produce it are the so-called " Coralligenous
Zoophytes " (Actinozoa Coralligena). The class of the
Actinozoa is one of the primary divisions of the sub-kingdom
of the Cxlenterata, or Radiated Animals ; and those forms
of the class which secrete a corallum belong to the orders
of the Zoantharia, Rugosa, and Alcyonaria. No corallum
is ever produced in the order Ctenophora, and many
members of the Zoantharia are either destitute of a skeleton
or have but an imperfect one, whilst the corallum of the
Alcyonaria is also often rudimentary. The Ctenophora,
therefore, will be wholly omitted here ; and only those
members of the Zoantharia and Alcyonaria in which a
well-developed corallum is present, will be considered at
any length, the soft-bodied forms requiring no notice
except in so far as they serve to elucidate the peculiarities
of the Coralligenous groups.`
The class of the Actinozoa comprises those Coelenterate animals in which the space included within the body-walls is divided into an inner " gastric " cavity or stomach and an outer " perivisceral " cavity. The gastric sac is short and is open below, so that the perivisceral cavity freely communicates with the outer world through the mouth, and the two cavities become practically one. The perivisceral cavity, in turn, is divided into a series of compartments by the development of a series of vertical membranous plates (the " mesenteries "), which are arranged in a radiating manner between the walls of the gastric sac and the wall of the body. The reproductive organs are not external, but are attached to the faces of the mesenteries, and shed their contents into the body-cavity. The ova reach the external medium either through the mouth or through the ends of the tentacles.
The class of the Actinozoa is divided into four orders, viz., the Zoantharia (Sea- Anemones, Madreporaria, &c.), the Rugosa (Cyathophyllum,Zaphrentis, (fee.), the Alcyonaria (Red Coral, Sea- Pens, Sea-Shrubs, Organ-pipe Corals, <fec.), and the Ctenophora (Beroe, Pleurobrachia, Venus s Girdle, &c.)
Order I.—Zoantharia (Hexacoralla).
The members of this order are distinguished by the fact that the intermesenteric chambers and tentacles are generally and fundamentally six in number, or some multiple of six, however largely they may be subsequently increased, whilst the tentacles are simple, rounded, or conical, not fringed with lateral processes. The corallum may be wholly absent (Actinidce), spicular (Zoanthidce), in the form of an internal axis or " sclerobasis " (Antipathidce), or " sclerodermic," with a distinct wall, and generally distinct septa (as in the Madrepores, and the " Stone-corals " generally). According to the nature of the skeletal struc tures the Zoantharia are divided into the three groups of the malacodermata, sclerobasica, and sclerodermata.
Zoantharia malacodermata.—In this section are the animals commonly known as Sea- Anemones, in which there is either no skeleton at all (Actinidce), or simply a discon tinuous pseudo-skeleton composed of minute adventitious spicules scattered through the integuments (Zoanthidce). Though possessing nothing that would ordinarily be termed a " coral," it may be well to insert here a general descrip tion of the anatomy of the Sea-Anemones, as they are readily accessible for study, and may be regarded as being fundamentally identical in the structure of their soft parts with the Coralligenous Zoantharia, as well (probably) as with the extinct Rugosa.
The true Sea-Anemones (Actinia, Tealia, Actinoloba, Sagartia, &c.) are under ordinary circumstances simple animals, but some closely allied forms (Zoanthus and Palythoa) form compound growths or colonies, which are produced by budding, and consist of numerous polypes united by a fleshy base or coenosarc. In the simple forms the body is generally a short cylinder or truncated cone (the " column "), usually of a fleshy or leathery consistence, and capable of undergoing great variations in shape. The lower extremity of the column usually forms a flattened dis- coidal area (the "base"), whereby the animal attaches itself at will to foreign bodies. The base, however, may be wholly non-adherent, pointed (Ilyavthus), thin, distensible, and imperforate (Edwardsia). or swollen, rounded, and pierced with a distinct orifice (Peachia and Cerianthus). The upper surface of the column constitutes a circular flattened area (the " disc "), which carries the tentacles round its margin, and is perforated centrally by the aperture of the mouth.
In size the Sea-Anemones vary from less than a sixth of an inch up to 2 feet in diameter ; and their habits of life are very various. Generally they attach themselves by the muscular base to foreign bodies, in rock-pools, at low- water mark, or extending to considerable depths. Others, again (such as Peachia, Halcampa, and Edicardsia), live more or less completely buried in the sand ; Cerianthus has the same habit, and further protects the body by the secretion of a loose, membranous, non-adherent tube f whilst Arachnactis is free-swimming and pelagic.
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Fig. 1.—Sea Anemones.
A, Edwardsia callimorpha, Gosso. B, llyanthus Mitchellii, Gosse, of the natural size. (After Gosse.)
layer (" ectoderm "), an inner layer (" endoderm "), and a more or less largely developed intermediate layer (" meso- derm "), each of which may in turn be differentiated into successive more or less distinguishable strata. The ecto derm is composed of an exterior stratum of ciliated epithelial cells, a granular stratum crowded with " thread-cells," and a stratum of pigment to which the brilliant coloration so characteristic of these animals is due. The peripheral epithelial cells are constantly being thrown off from the surface as a viscid mucus, which may entangle foreign bodies and form a species of investing tube. In other cases (Tealia, Bunodes), the surface is studded with adhesive
vesicular warts, by means of which the integument obtains