corals, the cells of the polypes are very numerous, and the coralline mass presents a surface beautifully marked with stellular impressions. The Astræa viridis is here represented as seen alive in the sea (Pl. VI. fig. 13). The polypes in this coral are of a dark-green colour, six lines in length, and are protected by deep, laminated, polygonal cells, two lines in diameter. They are striated with longitudinal and transverse bands, and connected by a fleshy layer which covers the dark-brown coral; some of the polypes are here shown expanded, and others contracted. In this magnified view (Pl. VI. fig. 10) of a single polype, the tentacula are seen expanded, or disposed around the prominent mouth. The appearance of groups of Astrææ, and other corals, when viewed while the animals are alive and in activity, is most beautiful; looking down through the clear sea-water, the surface of the rock appears one living mass, and the congregated polypes present the most diversified and vivid hues.*

The Pavonia are those corals which have deep and isolated cells, each containing a large depressed polype, very similar in its appearance and structure to the Actinia. Pl. VI. fig. 11, represents a group of cells containing polypes of the P. lactuca, from the shores of the South Sea islands. The polypes are of a green colour, and there is a connecting, transparent, fleshy substance, which extends over the extreme edges of the foliated expansion of this elegant zoophyte.

22. MEANDRINA CEREBRIFORMIS; or Brain-coral.-The large hemispherical corals, having the surface covered with meandering ridges and depressions, disposed in a manner somewhat resembling the convolutions of the brain, are well known by the name of the Brain-stone Coral (Lign. 144). In a living state the mass is invested with a fleshy substance, variously coloured, and having numerous, short, conical, polypiform, confluent cells, arranged in rows between the * Dana's Zoophytes, p. 29.

ridges. This zoophyte sometimes attains considerable magnitude; a very beautiful specimen in the British Museum is four feet in circumference. The base of the Mæandrina, like that of other corals, is adherent to the rock. As one

fleshy mass expires, another appears, and gradually expands, pouring out its calcareous secretion on the parent mass of coral; thus successive generations go on accumulating vast beds of stony matter, and laying the foundations of coralreefs and islands. We may compare, observes Sir C. Lyell,* the operation of the zoophytes in the ocean to the effects produced on a smaller scale on land by the plants which generate peat, where the upper part of the Sphagnum, or peat-moss, vegetates while the lower is entering into a mineral mass, in which the traces of organization remain when life has entirely ceased. In these Corals, in like manner, *Principles of Geology.

the more durable materials of the generation that has passed away serve as the foundation over which their progeny spreads successive accumulations of calcareous matter.

23. ALCYONARIAN ZOOPHYTES: GORGONIA, OR SEA-FAN.The Gorgonia flabellum, or Venus's fan (Lign. 145),is a flexible zoophyte inhabiting almost every sea, and frequently attaining a height of four or five feet. When fresh from the water it

is of a bright yellow colour. This species exhibits the usual structure of the corticiferous polypifera, or zoophytes having an internal axis or skeleton of a tough horny consistence, with an external envelope or rind entirely investing the former. The Gorgonia present great diversity of form and appearance. This specimen from the West Indies (Pl. VI. fig. 2) is remarkable for its richness of colour, being a bright yellow, spotted with red; this species (Pl. VI. fig. 3), from the Mediterranean, has its pendant branches very elegantly disposed, and is of a purplish-lake colour; in both these examples the axis is black, and of the consistence of tough horn. Another beautiful species from the Mediterranean (the Gorgonia patula of Ellis) is of a bright red, and has the openings for the polypes disposed in two rows; a portion, highly magnified, is here represented (Pl. V. fig. 3), and exhibits several polypes in different states of protrusion.

These flexible Alyconaria are mostly attached to the rocks by an extended base, the surface of which is usually deprived of the fleshy substance that invests the other parts. The stem which springs from the base, although in a few species simple, generally divides into branches, which are exceedingly various in their size and distributions :-double, single, anastomosed, pinnated, straight, and pensile; and the stems are either compressed, flat, angular, or cylindrical; but in all these modifications the same structure prevails—an axis and an external crust or rind. The former is either horny, elastic, flexible, brittle, or pithy, seldom stony, and of a dark

colour; the latter a soft or tough fleshy substance, studded with pores, from which the polypes issue; this rind becomes

LIGN. 145.-GORGONIA FLABELLUM; one-twentieth nat. size.

earthy and friable when dried. In the Isis, which may be described as a Gorgonia with a jointed stem, this structure is well displayed, as in this branch of Isis hippuris (Lign. fig. 3, p. 624), in which a portion of the cortical part is removed, and the axis exposed. In the water the various species present the most vivid hues of red, green, violet, and yellow. The Gorgoniæ inhabit deep water, and are found in every sea; but certain species appear to be restricted to tropical climates. The Sea-pens (Pennatula) have a fleshy feather-like polypary, with a bony axis, and live unattached, floating free in the water, or lying on the sea-bottom, more or less imbedded in sandy mud.

Numerous species of the Asteroid Polypes, especially Alcyonium, Gorgonia, Pennatula, and their congeners, have calcareous spicules * imbedded in their leathery or fleshy integument, as in the Sponges. Horny spicules have been discovered in some Actiniæ.+

24. THE RED CORAL; Corallium rubrum.-Among the Alcyonarian or Asteroid Zoophytes are a few genera having an axis composed of a calcareous stony substance; and one genus possesses a skeleton of so beautiful a colour, and susceptible of so fine a polish, as to be largely employed for ornamental purposes. The Red Coral is a branched zoophyte, somewhat resembling in miniature a tree deprived of its leaves and twigs. It seldom exceeds one foot in height, and is attached to the rocks by a broad expansion or base. It consists of a brilliant red stony axis, invested with a fleshy or gelatinous substance of a pale blue colour, studded over with stellular polypes. This figure (Pl. V. fig. 9) represents a branch of "Coral" with several polypes, highly magnified, as seen alive in the water. The cortical or fleshy substance is removed at the extremities of the branch, and the red stony axis exposed. As the cells of the polypes are only composed of the soft animal matter which rapidly undergoes decomposition after death, no traces of their structure remain on the durable skeleton.

The Red Coral, as is well known, is of a very hard and durable texture; it is obtained by dredging in different parts of the Mediterranean and Eastern seas, and forms an important article of commerce. It varies much in hue, according to its situation in the sea: in shallow water it is of the most beautiful colour, a free admission of light appearing necessary for its full development. It is of slow growth; eight or ten years, in a moderate depth of water, being necessary for it to reach maturity. Arrived at this period,

See Histological Catalogue, Roy. Coll. Surg. vol. i. p. 222, pl. 13; and Dana's Zoophytes, p. 53. + Histol. Catal. p. 226.