The tubes formed by these Polychaetes are very varied in constitution.[[345]] In some cases a mucus, which hardens to form a firm protective envelope, is secreted from special parts (e.g. the ventral gland shields of Terebellids and Sabelliformia), or from the greater part of the general surface of the body; in other cases the secretion serves to stick together particles of mud or sand, or shelly fragments, so as to form a more or less cylindrical tube (rarely branched), which is lined internally by the hardened "mucus," having the appearance of silk.
Fig. 151.—Clymene ebiensis in its tube (t) (from Règne Animal). a, Anterior, p, posterior end, which is, however, injured.
But the process of tube-making is not a simple one, for in many cases, at least, the worms exhibit definite powers of choice. Thus some species of Sabella choose only the very finest particles of mud; Terebella conchilega chooses fragments of shell and grains of sand; Onuphis conchylega employs small stones more or less of a size; Sabellaria makes use only of sand grains. Whilst some worms, like Terebella, Nicomache, and others, make a very irregular tube, Pectinaria builds a most remarkably neat house, open at each end, which it carries about with it, the narrow end uppermost (Fig. 152); the grains of sand are nearly all of the same size and only one layer in thickness, embedded in abundant "mucus," and with the outer surface quite smooth.
Sir J. Dalyell[[346]] made some most interesting observations on the method followed by sundry tube-formers in the building of their tenements, and these observations, though made nearly half a century ago, have required very little addition or correction in modern times. In speaking of Sabella, he writes as follows:—
Fig. 152.—The tube of Pectinaria auricoma. × 3. (From M‘Intosh.) This is its natural position as carried about by the animal.
Fig. 153.—The upper end of the tube of Terebella conchilega. Slightly enlarged. (From M‘Intosh.)
"Let a tall and ample crystal jar containing a Sabella be emptied of its contents and speedily replenished with sea-water; the animal, if in view, has retreated during the short interval; the orifice of the tube is closed, all is at rest. But soon after replenishment it rises, to display its branchial plume still more vigorously than before, and remains stationary, as if enjoying the freshness of the renovated element, always so grateful—the harbinger of health and strength to those whose dwelling is there. The passing spectator would conclude that he now beholds only a beautiful flower, completely expanded, inclining towards the light like some of those ornaments of nature decorating our gardens. He pauses in admiration. But if a drop of liquid mud falls amidst the element from above, disturbing its purity, then, while the plume unfolds to its utmost capacity, does the animal commence a slow revolution, the body also passing around within the tube. Now are the thousands of cilia fringing the ribs [i.e. the secondary filaments] of the branchiae discovered to be in vigorous activity, and their office to be wondrous. A loose muddy mass is soon afterwards visibly accumulating in the bottom of the funnel; meantime the neck or first segment of the body, rising unusually high above the orifice of the tube, exhibits two trowels beating down the thin edge as they fold and clasp over the margin, like our fingers pressing a flattened cake against the palm of the hand. [This refers to the lappets of the peristomial collar.] During these operations muddy collections are seen descending between the roots of the fans [right and left gills] towards the trowels, while another organ, perhaps the mouth, is also occupied, it may be, in compounding the preparation with adhesive matter. Still does the partial or complete revolution of the plume above, and of the body within the tube, continue; the bulk of the muddy mass diminishes, activity abates; it is succeeded by repose, when the tube is found to have received evident prolongation."