We have now to consider the Spiral Tissue under another aspect, _i.e._ that of acting as the internal support of an exterior membrane. Ringed tissues are necessarily conjoined with the Spiral, as they both discharge the same office, and in some cases merge almost imperceptibly into each other in the same specimens. This is most beautifully shown in the proboscis of the common House-fly, to which reference will presently be made. The subject is so large that only a comparatively small selection of examples can be made, the greater number belonging to Nature, and not to Art. We will first take the common movable Gas-lamp, with its accompanying tube. It is at present the tube of which we have to treat, the gas itself being reserved for a future page. It is necessary that, in order to enable the lamp to be moved from one spot to another, the tube through which the gas passes must be so constructed that if it be bent, or even coiled, it retains its form, and does not become flattened. In order to obtain this object, a very long thin wire is coiled spirally to a suitable length. Over this wire is sewn the casing of the tube, which is afterwards made waterproof with elastic varnish. A still simpler mode is by enclosing a spiral wire within a tube of vulcanised india-rubber. It will be seen, then, that by the elasticity of the spiral wire the tube must always retain its shape, no matter how much it may be bent. On the right hand of the illustration are shown the movable Gas-lamp and tube, and a portion of the latter is given with its spiral wire partially unwound, in order to show its structure. [Illustration: TRACHEA OF INSECT, WITH ITS SPIRAL THREAD.] [Illustration: TUBE OF GAS-LAMP, WITH ITS SPIRAL WIRE.] The large tubes which convey air to divers are made in the same manner, as they would not only succumb to the pressure of the water without the wire, but could not be dragged over obstacles or round corners without collapsing. It often happens that a diver is obliged, when surveying a sunken ship, to traverse the whole of her interior, descending ladder after ladder, and entering every cabin in the ship. This could not be done but for the internal coil of wire within the tube. Reference will presently be made to the subject of diving. * * * * * ON the left hand is seen an object that looks something like a branch hollowed very thin. It is a magnified view of part of the Trachea or breathing-tube through which air is conveyed into the system of an insect. These breathing-tubes ramify to every portion of the body of an insect, even penetrating to the extremities of the antennæ, the wings, and the legs. It is obvious that as these organs are in tolerably constant movement, and the legs are much bent at every joint by the action of walking, the air-tubes which run through them must possess the same qualities as those of the gas-lamp and diver.