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A, Daisy anemone: o, mouth; r, tentacles. B, diagram of internal structure, horizontal section: a, septa leading from external wall to stomach; b, b, c, c, inferior septa; s, stomach.
have been, and it is to be hoped has been, done within the last few years, through Dana's delightful book and the many reviews of it which have appeared. Corals proper are the result of three different kinds of animal life, the true corallines being vegetable in their nature. They are not, however, made in any true sense of the word, but form about or within the living tissues of the organism, as the shell of the mollusks and the skeleton of the vertebrates form by the unconscious processes of secretion, absorption and transmutation. some cases this skeleton is external or shell-like; usually it is internal.
Secretion is a process which is best performed by the lower organisms; as Dana says, "The very simplicity of their structure enables them to carry on the processes of growth and nutrition, while they are almost all stone." Of the three groups of coral-making animals, only two will be considered here. The Actinia, or sea-anemones, and the Hydroida. The third group, the Polyzoa or Bryozoa, belongs to the sub-kingdom of the mollusks, and will be the subject of the next article in this series.
The coral polyps, while showing an organization far above the rhizopods, are equally destitute of mucous, circulatory, and of a complete digestive system. The fluid which performs the function of blood is merely digested material mixed with seawater. This distributes itself through all the interior cavities of the body, the undigested portion being ejected through the mouth, or pores of the skin. In species which live buried in the sand, an approach to gills for the aeration of the circulating fluids is found; but these are not coral polyps. A curious example of rudimentary organs of sense is found in some of the more highly organized actiniæ. Arranged around the stem of the polyp just below the tentacles is a chain of eyes, each with a crystalline lens and short optic nerve. But each nerve is isolated; it leads nowhere and is connected with no nervous ring or nervous center. Still it may be said that these creatures see in some dim fashion; they are sensitive to the action of light, besides giving other evidences of the power of vision.
Of the coral-making polyps the most important are the actinoid [Fig. 1, A]. The highest forms of these, though they do not "clog vital action and prevent all locomotion" by the secretion of coral, are essentially the same as the true coral polyps. The external form of these sea-anemones is somewhat like that of a garden aster, which they also resemble in beauty of coloring. The flower-like disk, with its fringed border of tentacles, is supported by a thick pedicel of almost equal diameter with the disk. In the free anemones there is at the lower extremity another disk for purposes of at
FIG. 2. CARYOPHYLLIA SMITHIL.
A, Tentacles expanded; B, tentacles closed. [After Gosse.]
tachment as well as of locomotion-slow though their locomotion is; in certain forms this is replaced by a bulbous-like termination with which they burrow in the sand and anchor there. In the center of the upper disk is an orifice or mouth which
leads into the stomach beneath. In many varieties the stem is studded over with warty protuberances, which act as suckers, and serve the purpose of attachment; in others it is marked by deep corrugations or transverse wrinkles, the result of contraction. The sac-like stomach of the actinia occupies only the upper portion of its pedicel; but the whole stem, both above and below the stomach, is radiate in its structure. The body [see Fig. 1, B] is divided, by fleshy partitions extending from the circumference toward the center, into narrow, wedge-shaped compartments, each one of which communicates at the top with the hollow of a tentacle. The more important of these septa [a, a] reach from the outer wall of the pedicel to that of the stomach, which they serve to support in its proper position, as well as to dilate it for the reception of food. A number of intermediate sepia [b,b, and c,c] do not reach so far inward as the stomach walls, but serve to render the body in the highest degree contractile. Besides the muscles in the septa, the body is furnished with two other sets-one circular, the other longitudinal; there are also others in the tentacles. By means of this elaborate muscular system, the polyp, when disturbed, immediately ejects the water from its body through the mouth and the numerous pores in its exterior walls; the whole disk with the tentacles surrounding it is drawn in, and the sides of the stem are rolled up over these. The appearance is then merely that of a rounded or conical lump [Fig. 2, B]. The disturbing cause being removed, the water is again admitted mainly through the mouth, and the flower slowly opens out and spreads abroad its petal-like tentacles [A].
In their external form and outward structure, in their organs for catching and appropriating their prey, in their powers of digestion and of respiration,-the free and the coralproducing polyps are alike. What constitutes the main difference between them is that the free actinoid polyps generally develop by ova, and the corals by budding; but especially that the latter secrete from the sea-water, which circulates throughout their bodies, the mineral substance of the coral.
This mineral substance-every component of which has been detected in seawater-is deposited between the septa already described, and is therefore radial in its structure; of course, this coral skeleton does not extend into the stomach or disk,as this would hinder the performance of the vital functions, but is confined to the septa
of the sides and to the lower part of the body. When the polyp is alive, the top, and usually the sides, of the coral are concealed by the disk and tentacles, and by the skin of the living animals. In higher animals, as the skeleton receives new accretions, the older particles pass away, so that, after maturity is reached, the skeleton, under normal conditions, remains of a constant size. The coral polyps, however, after a time secrete more skeleton than they can accommodate, and so grow up and away from their mineral frame, leaving its stem below bare of any vital tissue, the flowerlike polyp crowning the tip alone.
The prey of this curious animal-flower is captured partly by means of its tentacles,
a, b, c, d, e, Successive stages of development; /, perfect lasso coiled in its cell; g, lasso shot out, only one-fourth of thread showing.
which, however, are usually too short to be quite sufficient for this purpose. There are, however, several ways in which the action of the tentacles is aided in the performance of their function; sometimes the whole disk rolls over the captured animal and forces it down into the digestive sac. The mouth and stomach of the actiniæ are so extensible that they frequently swallow animals as large as themselves. In such cases, the margin of the mouth gradually extends itself till it is able to close over the victim,—a large oyster or crab it may be,-until finally the creature with its shell is fairly engulfed. The nutritious portions having been absorbed, the anemone rids itself of the indigestible débris by the simple process of
turning itself inside out,-which apparently | far beyond the body of the animal; their causes it not the least inconvenience.
lassos, moreover, are exceptionally long. The tubular threads measure from one-fifth to one-tenth of an inch in length, their cells being from one-twentieth to one-fortieth as
around the mouth and over the surface of the tentacles are myriads of tiny cells [Fig. 3,f], each one of which contains, coiled up in its cavity, a long, slender, hollow thread, which the animal can shoot out at will with lightning-like rapidity. This is effected by the turning of the tubular filament inside out, like the finger of a glove, the tip being the last part emitted. Somewhere in connection with the basal part of this lasso is a receptacle for poison, which is injected into the wound made by the dart. Once ejected, the lasso is henceforth useless to the actinia, -a matter of no consequence to the polyp, however, since its magazine is so abundantly furnished, and is moreover constantly being replenished [Fig. 3, a, b, c, d, e]. In the internal cavity of the body, along the free edge of the septa, and attached to a membrane, which is very much plaited or ruffled,
long. The thread under high powers looks like a twisted cord of two strands, thickly beset with hairs or bristles for some distance above its base [Fig. 3, g]. These same lassos, or cnida, are found in the other family of coral-making polyps, the hydroids.
When any animal not protected by a shell is captured, death ensues very soon, and upon examination, its flesh is found to be penetrated by the lassos. Gosse mentions an instance that he witnessed, of a small fish which had previously been swimming vigorously about, and which died in great agony a few minutes after the momentary contact of its lip with the lasso-studded surface of a sea-anemone; the lip, when examined, was found to have been pierced by the poisoned darts of the actinia.
These creatures, which form the vast masses of masonry that support the palmcrowned islands of the Pacific, are usually of very minute size, as the markings upon any common specimen of coral will show. Some of the coral-clumps of Oceanica
is a slender white cord; these cords are found in bunches about the septa, and are extended through the mouth or through the basal orifices, which are apparently left for this purpose, and also through any ruptured portion of the skin. About these acontia, as they are called, are lasso cells arranged radially and in myriads; these aid greatly in the destruction of prey, as they extend
HEAD OF SERPULA. [FROM NATURE.]
which have come from a single progenitor measure twenty feet in diameter, while the individual polyps which have formed it have
an extreme length of not more than one twentieth of an inch. The form of the mass of coral, or corallum, is determined by the mode of propagation by which the colony has been developed from the original progenitor, the forms resulting from multiplication by gemmation, by budding, by self-division, and by fission, being very different. The polyps rarely lead a single life; an individual sends out from its stem a bud, which soon develops a mouth and tentacles of its own, though still adhering to the parent organism. Another and another bud is sent out; the branches divide and subdivide by repetition of this process, until the compound organism assumes plant-like form, though in nature strictly animal. In many varieties, an irregular circlet of buds crops out below the disk of the parent form; others develop in their turn as the stem grows upward and away from the first set. It follows that c
FIG. 6. MADREPORACEAE.
the parent form is always at the tip of the stem, its oldest offspring being next the base, and the rest arranged above in order of seniority. Here and there upon the lengthening stem, a single polyp begins a career like that of the patriarch of the colony, and so finally a perfect coral-tree is formed, each branch being the result of thousands of individuals. The mass grows upward until the sea-level is reached, and then it frequently continues to grow radially, forming enormous flat disks sustained by a central stem, which is sometimes slender enough to give way beneath the pressure of a ship's keel from above.
The compound organism, whether formed by budding or by division, does not lose its continuity; throughout their entire existence, the union of the individuals remains of the most intimate kind: though each polyp may claim as private property, a mouth, a set of tentacles, and a stomach, these still capture, devour and digest for the public good. The circulation of digested matter in solution extends throughout the entire system. The Zoöthome is indeed a sheet of living animal matter, fed by innumerable mouths and nourished by as many stomachs. Nevertheless, the compound body of the colony is subject to the same cycle of change which families and races everywhere undergo. While the living sheet is renewing itself in one place it is dying off in another. Strange to say, however, the parent form in many varieties goes onward and upward,
thus solidify the rock they are forming by filling the pores as their tissues dwindle. By degrees, as the mass increases in size, these die off, and finally not only the lower portions, but also the interior of the structure, is mere dead skeleton [Fig 41. While the coral rock is solidifying below, the upper and exterior portions are clothed with a layer of living animals. The surface is secured against the wear of the waters by an impervious deposit of carbonate of lime, secreted along the edges of the dying polyps; it is further protected by numerous minute incrusting species of coral. Certain barnacles attach themselves as they do to the ship's bottom, without injury however, to the stability of the mass. Many of the serpulæ attach themselves in the same way (according to Dana), the coral having the same rate of growth as their own. The zoöphyte grows around these, leaving a tube which reaches deep into the coral rock. When alive, the serpulæ expand a "brilliant circle or spiral of delicate rays, making a gorgeous display among the coral polyps " [Fig. 5].
When a piece of coral rock is suddenly broken off the whole colony feels the shock;
FIG. 7. A, Coral with polyps extended. B, Polyp magnified. C, Longitudinal section of polyp with its tentacles withdrawn ; a, apertures
in tentacles; o, ovary; s, spermatazoa. (After Dana.]
the tentacles of each polyp close at once. But these animals peculiarly understand making the best of a bad case, and in a short time, all is right again; even the polyps which were torn repair their loss and soon regain their pristine size and power, while under favorable conditions the broken fragment forms the germ of a new colony.
The solid, dome-shaped masses frequently seen in collections are produced by polyps which propagate by division, a common mode in all low forms of life, whether animal or vegetable. The disk elongates into an oval, and a new mouth opens beside the first; longitudinal division takes place in the stem, and tentacles sprout from the recently adherent edges; two individuals springing from a common base are thus formed. Such is a simple case of fission. By extension of the process certain branching forms [Fig. 6] result, as well as the magnificent clumps called Astræas, which extend over such vast areas of tropical sea-bottom.
The Meandrina, sometimes called "brain coral" from the tortuous lines, suggestive of the convolutions of the brain with which it is marked, is produced by a modification
of the process above described. In this polyp the round disk elongates enormously, one mouth forms after another, until whole rows exist side by side before fission occurs; when it does take place, each individual in its turn propagates a new row.
The Caryophyllia smithii [Fig. 2, A, B] represents one of the most beautiful of the few solitary species of coral polyps. Beautiful as is the form of this creature, no uncolored drawing can give an idea of its appearance. When filled with water and its tentacles spread, it is delicately translucent throughout; the star surrounding the serrated mouth is either of a deep, rich brown, a vermilion, or an emerald green color, brilliant as the breast of a hummingbird. The knobbed ends of the tentacles are either opaque white, or tinged with rose-color. Seen through the lens and under water, the creature seems some exquisite dream of a zoöphyte rather than solid flesh. In one variety the brighter tints give place to an opalescent play of color. These polyps are evidently sensitive to light, though they have no eyes. They very long-lived, a single aquarium specimen having been kept for two years and a half after its capture. They reproduce, like the single actinia, by ova, which escape, form into a polyp, and then, before beginning the secretion of coral, multiply by self-division, each individual usually separating from the parent stock. The lassos of this species, Gosse tells us, are as elastic as steel, more like wires of that material than threads, yet readily turning inside out; it is not stated whether this elasticity is peculiar to the species.
Mention has already been made of the manner in which the actinoid polyps secure their food, and of their extraordinary capacity for disposing of it when caught. There is, indeed, something ludicrous in the pertinacity with which the central idea of the actinia is retained under all conditions and in every mode of reproduction, which invariably looks first of all to the attainment and multiplications of mouths, and, of course, their necessary concomitant, stomachs.
An amusing instance is mentioned by Gosse of voracity in a coral polyp which he had transferred to his aquarium. basal portion had been torn away from the coral below, so that at least four-fifths of it was free, and still held away from any solid foundation. Happening to examine it some days after, he discovered that the