The _Tubipora_ is a calcareous coral, formed by a combination of distinct, regularly-arranged tubes, connected together at regulated distances by lamellar expansion of the same material. The aggregate formation resulting from this combination of tubes constitutes a rounded mass, which often attains a very considerable size. In Fig. 41 we have a representation of the zoophyte _Tubipora musica_ and its product, which is sometimes designated by the vulgar name of Sea-Organ. In the engraving, 1 is the calcareous product, reduced to half its size; 2, is a portion in its natural size; 3, the tubes magnified, and containing the polyp which occupies the summit of the tube, the whole of which constitutes this curious coral; 4, is the polyp magnified; 5, the head or collection of tentacula of the individual polyp. Zoologists of the last century confounded all the species of this genera inhabiting the tropical seas, making only one species, to which they gave the name of _Tubipora musica_. But it is now known that there are many species of _Tubiporæ_, readily distinguishable in a fresh condition by a difference in the colour of the polyps. The tissue of these singular beings is of an intensely red colour. The disposition of their tubes in the style of organ pipes has always attracted the attention of the curious inquirer into the secrets of Nature. II. GORGONIADÆ. Milne Edwards divides this order into three natural groups:--I. The _Gorgoniadæ_. II. The _Isidians_. III. The _Corallines_. [Illustration: Fig. 42. The Sea Fan, Gorgonia flabellum (Linn.).] The _Gorgonians_ are composed of two substances: the one external, sometimes gelatinous and fugitive; sometimes, on the contrary, cretaceous, fleshy, and more or less tenacious. Animated with life, this membrane is irritable and encloses the polyp; it becomes friable or arenaceous in drying. The second substance, internal and central, sustains the first, and is called the _axis_. This axis presents a horny appearance, and was formerly believed to possess chemical characters analogous to the horns and hoofs of some of the vertebrated animals. It has recently been asserted that the tissues of these corals consist essentially of a particular substance which resembles horn, but which is called _Corneine_. A little carbonate of lime is sometimes found united with this substance, but never in a sufficient quantity to give it a stony consistence. This outer covering developes itself in concentric beds, between the portion of the axis previously formed and the internal surface of the sclerotic covering. [Illustration: Fig. 43. Fan Gorgon, magnified.]