These general remarks may appear obscure and insufficient to the larger number of our readers. They are necessarily so; they are generalities upon animals very little known, even to naturalists. We quit this difficult ground, trusting to make the special study of the several types we shall have to describe more interesting. The group of Polyps divide themselves into many classes, namely, the _Alcyonidæ_, the _Zoanthina_, the _Discophora_, and _Ctenophora_. It will be our task to describe in succession the habits and characters of each of these classes, dwelling on such species as appear to us to offer to the reader most real interest. FOOTNOTES: [Footnote 5: "Outlines of Comparative Anatomy."] CHAPTER VI. CORALLINES. "As for your pretty little seed-cups or vases, they are a sweet confirmation of the pleasure Nature seems to take in superadding elegance of form to most of her works. How poor and bungling are all the imitations of art! When I have the pleasure of seeing you next, we shall sit down--nay, kneel down--and admire these things."--HOGARTH TO ELLIS. The Alcyonaria are so designated from their principal type, that of the Alcyons. The fresh-water species are composed of a fleshy, sponge-like mass, consisting of vertical, aggregated, membranaceous tubes, which are open on the surface. In these tubes the polyps, which are Isidians, are located. The mouth is encircled with a single series of filiform tentacula, which, like those of the whole family, are depressed or incomplete on one side. The eggs are contained in the tubes, and are coriaceous and smooth. The tentacula of these polyps are generally eight, disposed somewhat like the barbs of a feather, and toothed on their edges like a saw, which has procured them the name of _Ctenoceros_, from the Greek word χτεις, a comb. Their bodies present eight perigastric lamellæ; their coral is often formed of spiculæ. We shall see, farther on, that among the Gorgonidæ the coral ceases to be parenchymous--that is, spongy and cellular; that its axis assumes a horny and resistant consistence, which becomes stony in the corallines. In this last group, the external bed, which is the special lodging of the polyps, always remains soft on the surface. We shall have a general idea of the organization, manners, and mode of multiplication among the Alcyonaria when we come to treat of corals and their strange history. The class Alcyonaria is divided into many orders. We shall consider--I. The _Tubiporinæ_. II. The _Gorgoniadæ_. III. The _Pennatulidæ_. IV. The _Alcyonaria_, properly so called. I. THE TUBIPORINÆ form a group consisting of several species, which live in the bosom of tropical seas, in which the Coral Islands form so prominent a feature. The group is exclusively formed of the curious genus _Tubipora_. [Illustration: Fig. 41. Tubipora musica (Linn.), half the natural size.]