The tubes themselves are worthy of study and preservation, more especially as they vary in form, and may, to some extent, provide a means by which the different species may be identified. They are composed of fine layers of calcareous matter secreted by the body of the worm within, and lined by a thin leathery membrane which may be easily exposed by dissolving away the mineral matter as just described. Some are triangular in section, and often distinctly keeled, while others are cylindrical, and flattened more or less on the lower side. The triangular tubes are attached to stones or shells throughout their length, but the cylindrical ones are often elevated above the surface in the wider and newer part. If a cluster of these tubes, freshly gathered from between the tide-marks, be placed in the aquarium, the worms will soon protrude the foremost portion of their bodies, exposing beautiful fan-like gills, often brilliantly coloured in shades of scarlet, blue, or purple, which are kept in motion in such a manner as to convey water, and consequently also food, towards the mouth. The gills are of course, richly supplied with blood, for their main function is to aërate that liquid by exposing it to the water in order to absorb oxygen gas. The body of the worm is provided also with little cilia, which, by their constant vibratory motion, keep up a circulation of water through the tube; and this not only keeps the tubular home free from excrement and other sedimentary matter, but also probably assists in the function of respiration by bringing fresh supplies of water in contact with the animal’s soft and absorbent skin. [Illustration: FIG. 122.--_Serpula_ REMOVED FROM ITS TUBE] When the worms are disturbed they immediately withdraw themselves within the tubes, this being done by the aid of the numerous minute hooklets on the surface of the body that enable the worms to cling firmly to the membranous linings of their homes; and it will then be observed that the mouth of each tube is closed by a lid (_operculum_), which hangs as by a hinge when not in use. These operculi vary much in character, and supply another aid in the identification of the various species. They differ much in shape, and may be either membranous, horny, or calcareous. Little calcareous tubes, somewhat similar to those of the _Serpulæ_, but always in the form of a spiral, may often be seen on stones and shells, and the fronds of sea weeds, sometimes so closely packed together as to almost entirely cover the surface. The average diameter of these spirals is only about a sixteenth of an inch, and many are so small that a lens is necessary to discern their shape. In general form they closely resemble some of the small species of _Planorbis_ shells that are so common in our ponds and streams, but these latter are the shells of freely moving _molluscs_, and are generally of a brownish colour. [Illustration: FIG. 123.--THE SEA MAT (_Flustra_)]