_Zoantha thalassanthos_ (Lesson), which has given its name to the group, consists of large turf-like tufts of coral attached to a rock. Its animalcules are packed closely together, and their expanded flower-like heads have a curious resemblance to a mass of flowers in full bloom. They are borne on bending root-like stems of pure white, interlacing one with the other, surmounted by a fusiform or spindle-shaped body, pediculate and swelling towards the middle, but truncate at the summit, of a reddish-brown colour, marked with longitudinal stripes more highly coloured; its consistence is firm and parchment-like. From the body issues a tube, narrow, muscular, contractile, and red in colour, terminating at the summit in eight elongated arms or tentacula, of a pure yellow, traversed by a nervure of the same colour. The edges of these arms are fringed with fine pinnæ, parallel to each other, of a bright maroon colour, and resembling the barbs of a feather. According to Lesson, the arms of this _Zoantha_ are kept unceasingly in motion, which produces in the water small oscillating currents, in the course of which the animalcules on which the polyps feed are precipitated into the stream leading to their mouths. The tendency to produce a calcareous polypidom is a property almost universal with animals of this class. Zoologists are agreed in dividing them into three very distinct orders--namely, the ANTIPATHIDÆ, consisting of the genera _Antipathes_, _Cirripathes_, and _Seipathes_, in which the polypidom is of a horny consistence; the MADREPORIDÆ, in which the polypidom is calcareous and stony; finally, the ACTINIDÆ, which produce no polypidom. ANTIPATHIDÆ. We need not dwell upon this group, which is comparatively uninteresting. They correspond with the family of _Gorgonidæ_ among the _Alcyonaria_, which they resemble in having the central axes branching after the manner of a shrub; but the polyps have the mouth surrounded with a crown of six simple tentacula. The axis is of a harder and denser tissue than that of the Gorgons, and presents on its surface small spiniform projections. The polypiferous crust, with which they are covered, is in general very arenaceous, and is so easily detached, that it is rare to see in collections anything but the denuded skeleton of the colony. In _A. arborea_, the polypidom is fragile and brittle; when dry, the branches, always slender and delicate, resemble the barbs of a feather. The colour is of a deep black, or rather bistre and _terra de sienna_ tint. Under a powerful lens, the extremities of the branches appear to be covered with small spines, and the trunk is formed of oval and irregular concentric beds, which are the zones of growth. Its consistence is firm, so that it can be worked up and converted into chaplets for pearls and other bijouterie: it is known in commerce as _black coral_. MADREPORIDÆ.