Animal protected by a tube; locomotive organs in the form of foot-tubercles, carrying setae ; breathing-organs in the form of branchiae carried on or near the head. Sexes almost always distinct. A metamorphosis in development. The Annelides which are included in this order inhabit tubes, which may be calcareous, and secreted by the animal itself, or may be composed of grains of sand or pieces of broken shell, cemented together by a glutinous secretion from the body. The body-rings are mostly provided with fasciculi of bristles set upon lateral foot-tubercles or para-podia, by means of which the animal is enabled to draw itself in and out of its tube.
The Tubicola are often united with the order of the Errantia under the name of Polychaeta ; and though collectively spoken of as " Branchiate Annelides," they do not always possess specialised respiratory organs.
The protecting tube of the Tubicolous Annelides may be composed of carbonate of lime (Serpula), of grains of sand (Sabellaria), or of sand, pieces of shell, and other adventitious particles cemented together by a glutinous secretion from the body (Terebella); or it may be simply membranaceous or leathery (Sabella). Sometimes the tube is free and non-adherent (Pectinaria); more commonly it is attached to some submarine object by its apex or by one side (Serpula and Spirorbis). Sometimes the tube is single (Spirorbis); sometimes the animal is social, and the tubes are clustered together in larger or smaller masses (Sabellaria).
When the tube is calcareous, it presents certain resemblances to the shells of some of the Molluscs, such as Vermetus and Dentalium. In the living state it is easy to make a distinction between these, for the Tubicolar Annelides are in no way organically attached to their tubes, whereas the Molluscs are always attached to their shell by proper muscles.
The pseudohaemal system has its usual arrangement, and the contained fluid is usually red in colour, but is olive-green in Sabella. The respiratory organs are in the form of filamentous branchiae, attached to, or near, the head, generally in two lateral tufts, arranged in a funnel-shaped or spiral form. Each filament is fringed with vibrating cilia, and the tufts are richly supplied with fluid from the pseudo-haemal system. There is no special apparatus required to drive the blood back to the heart, but this is effected by the contractile power of the gills themselves. From the position of the branchiae upon, or near, the head, the Tu-bicola are often known as the "cephalobranchiate" Annelides (fig. 130).
Reproduction in the Tubicola is generally sexual, the sexes being almost invariably in different individuals; but fission has also been noticed to occur. As regards their development, the young pass through a distinct metamorphosis. The larvae (fig. 131, A and D) are freely locomotive, furnished with eye-spots, and swimming actively by means of cilia, which are principally aggregated into two rings or circlets, one placed on the head, the other at the hinder end of the body. The tentacles are developed at an early period, and the larva undergoes segmentation. Finally, the cilia disappear, the larva becomes stationary, and the protective tube of the adult is secreted. The young Tubicolar Annelide thus resembles the permanent condition of the Errant forms; and the stationary condition of the adult, accompanied by the loss of its sense-organs, may be regarded as an instance of "retrograde development."
Fig. 130. - Tubicola. a Serpula con-tortitplicata, showing the branchiae and operculum; b Spirorbis communis.
Fig. 131. - Development of Tubicolar and Errant Annelides. A, Larva of Terebella; o Position of the mouth ; a Anus, surrounded by the posterior circlet of cilia; c Anterior circlet of cilia ; t Tentacle. 13, Polytrochal larva of Arenicola; C, Larva of Pnyllodoce. D, Larva of Spirorbis; t t Tentacles. All the figures are greatly magnified. (After Claparede, Schultze, and A. Agassiz.)
The most familiar of the Tubicola is the Serpula (fig. 130, a), the contorted and winding calcareous tubes of which must be known to almost every one as occurring on shells or stones on the sea-shore. One of the cephalic cirrhi in Serpula is much developed, and carries at its extremity a conical plug, or operculum, whereby the mouth of the tube is closed when the animal is retracted within it. The operculum of Serpula has a more than ordinary interest in the fact that it is the only instance in the Annelida in which calcareous matter is deposited within the integument. In Spirorbis (fig. 130, b) the shelly tube is coiled into a flat spiral, one side of which is fixed to some solid object. It is of extremely common occurrence on the fronds of sea-weed and on other submarine objects.
Equally familiar with Serpula is Terebella, the animal of which is included in a tube composed of sand and fragments of shell, cemented together by a glutinous secretion. In the Sabellidae the tube is composed of granules of sand or mud. In Pectinaria the tube is free, membranous, or papyraceous, covered with sand-grains, and in the form of a reversed cone of considerable length. In Phoronis, the tube is membranous, and the branchiae are carried upon a horse-shoe-shaped process, which is strikingly similar to the "lophophore " of the Polyzoa.