or resembling rude hillocks, thin plates even or nodulous, foliaceous expansions, arborescent growths, nearly simple, or highly compound; and very many other forms: all of these may be recognised as the result of certain peculiarities in the coral animals, and chiefly in their mode of increase by gemmation or budding, which may be either superior or inferior.

In illustration let us take the polyp with which we are most familiar, the common sea anemone, which though it secretes no corallum is yet very similar to many of the true coral polyps. The upper extremity of the anemone with its wreath of tentacles is called the disk, in the centre of which is the mouth. If in the coral polyp a fresh mouth appear in the disk, indicating the commencing growth of another polyp, the budding is said to be superior or terminal.

If a transverse section be made of a sea anemone the interior is found to be divided by membranes radiating like the spokes of a wheel; alternating with these in the coral animal are calcareous lamellæ, (Fig. 1,) but these frequently extend on all sides beyond the visceral cavity: should the growth of a fresh polyp proceed from these external portions of the lamellæ the budding is considered, as before, to be superior.

FIG. 1.

But when the base of the new polyp is connected with the base or lower portion of the parent polyp, the budding is said to be inferior. The growth of a polyp may be acrogenous in the direction of its length at the apex, (never at the base,) or it may be lateral in the direction of its breadth. These distinctions together with those relating to its mode of reproduction by superior and inferior budding will be found sufficient to account for the wondrously diversified forms presented by the stony corals.

Of the three principal groups into which coral zoophytes are divided the first is

ASTREACEA.

In the genus Fungia, or mushroom coral, we have a thick circular or oblong disk, more or less convex above: the lamellæ radiate from a central depression answering to the mouth of the animal, and at the circumference are not bounded by any transverse partition or cell wall. The whole corallum is the framework of a single polyp, by the jelly flesh of which it is entirely covered. As the lamellæ diverge the interstices are longitudinally divided by shorter lamellæ, and from the point where each of these reaches the common height, a tentacle springs, so that in the place of the wreath surrounding the disk of an actinia, the whole disk of the Fungia polyp is covered with scattered tentacles. In the Fungia there is no increase by budding; ovules are given off which are at first free and endued with the power of locomotion, these soon become attached, often to the concave or under side of the parent; as growth proceeds they again become free, and are in a living state generally found resting amongst the branches of other corals. The colour of the animal is umber, with green tentacles: it is probable they have a limited power of motion.

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In Fungia the animal is perfectly simple, but in Herpetolithus or the slug coral, the disk includes many polyp mouths, and in Polyphyllia or the mole coral, it is made up of a vast number of centers with short radiating lamellæ yet the character of the Fungida is preserved by the absence of any partition between the centers; the lamella of one center uniting with those of the surrounding centers. The single visceral cavity being a distinctive mark of a polyp, we cannot consider the mole coral as a single highly compound animal, yet the junction of the polyps is so intimate that it is impossible to trace where one polyp begins or another ends.

Hitherto there has been a remarkable resemblance in form between the species examined: but the same absence of cells, the same intimate connexion between the polyps is observable in many Fungidæ having an entirely different aspect.

In Pavonia we have ascending thin clustered leaves gracefully spreading, or intersecting and coalescing so as to form a mass with angular or contorted intervals. The upper surface of the folia is covered with long and beautifully fine lamellæ radiating from star-like oviremes or polyp mouths: in some species both surfaces appear thus furnished, and the coral is termed bifacial, but this probably arises from the folding back of the leaf upon itself.

In Agaricia the form is a dimidiate plate, or sometimes resembles a delicate tazza vase on a short pedicel. This genus has been by Ohen much more aptly named Undaria, from the rippled or wave-like ridges covering the surface. The thin plate-like form of these Fungidæ is produced by the lateral growth of the polyps. If a simple polyp has acrogenous growth, increasing in the direction of its length, without budding, it forms a stem or column, the lower part closing as additions are made at the apex: if a number of such polyps grow side by side, a thick mass is produced: but when a polyp spreads itself horizontally only, its corallum is wafer-like, and if budding takes place near the circumference, the same thin flat form is extended, and is usually found encrusting other substances. But it frequently happens that polyps with this kind of lateral growth extend themselves not horizontally, but vertically, or in an ascending direction, thus forming a leaf-like corallum in which the axes of the polyps are in a horizontal position and the coral grows upward by their lateral increase.

In the classification of corals, the mere shape of the corallum is considered to be of little importance: thus in the Fungidæ we have thick circular or oblong disks, erect leaves,

or thin plates encrusting other corals, the distinguishing mark being the uninterrupted continuance of the lamellæ from one polyp mouth to another. Sand-like or granular projections called by Milne Edwards synapticules, very frequently beset both sides of the lamelle in the genus Fungia.

In passing to the very extensive family Astræidæ, the first thing which may strike us is the circumscribed character of the cells; they may be large or small, circular, oblong, or greatly lengthened and sinuous, still they are enclosed by cell walls: no less obvious it is that these cell walls do not confine the lamelle, which project through them, and into the spaces beyond them.

Most of the Astræidae exhibit a tendency towards a convex or hemispherical form, no other being so well adapted to remain uninjured in the dashing of the surges which perpetually beat upon the borders of the coral reefs. Let us now suppose a cluster of polyps forming a small hemisphere, and let it be required to contrive for them a mode of growth, which, without any change in the size of the polyps, shall preserve the form of the zoophyte unaltered, whilst the hemisphere of a few inches in diameter increases to one of many feet. Nature in the Astræidæ has practically solved this problem, not in one or two ways only, but in a vast variety, all equally admirable, for beauty of appearance and mechanical perfection. In some species the whole zoophyte is massive, the dome-like surface being nearly even; but in others the polyps that have grown originally from one parent do not coalesce with other surrounding families of polyps, and hence deep intervals are left between the calicles.

If a number of polyps occupy the surface of a hemisphere, each one having simple acrogenous growth, it is plain that in course of time their calicles will diverge and at their extremities become separated by wide intervals; in this way losing both space and strength; in the Astræidæ this is

obviated by disk budding. The simplest case of disk budding is when the circular disk of a polyp with its fringe of tentacles lengthens into an oblong form; the polyp mouth also in the centre of the disk becomes elongated and at length divides into two distinct mouths separated by an interval; across this interval a double fringe of tentacles is at length formed, and thus in place of one, two distinct and perfect cells arise (fig. 2).

If with no further budding these two cells grow upward, a forked calicle is produced with cylindrical branches.

But if the parent cell instead of forming two distinct and similar cells widens continually as it ascends, we shall have a calicle in the form of a compressed funnel. The number of lamellæ occupying an inch of cell wall seldom varies in the same species, nor does the number of lamellæ surrounding a single polyp mouth. As compared with the parent cell, the compressed funnel-shaped calicle must therefore include a vastly increased number of lamellæ, and a proportionable number of polyp mouths.

These two forms then, the forked calicle with cylindrical or turbinate branches (fig. 3), and the compressed funnel form, may be considered the types of growth in the segregate Astræidæ, that is in those species in which the calicles from separate parent cells do not altogether coalesce.

It is, however, very seldom these forms are found quite