In the Terebella there are distinct organs for the aëration of both liquids, which form a beautiful plume when expanded, as in fig. 133, which shows the animal when out of its tube. What may be called the head is fixed upon the first ring of the body. The mouth has a lip like a funnel-shaped cup with numerous long slender tubular tentacles; and two delicate arborescent branches or gills are fixed immediately behind the head. The colourless liquid which occupies the space between the alimentary canal and the ventral wall of the worm, is sent by the contractions of the body into the slender tubular filaments round the mouth, which are covered by cilia, whose action continually renews the stratum of water in contact with them. The blood in its usual course enters the capillary tubes of the arborescent gills, where it is oxygenized, and, after being distributed to the different parts of the body, returns to the heart and gills again. [Illustration: Fig. 133. Terebella conchilega.—_a_, lip, surrounded by tentacles, _b b_, all placed upon the first segment of the body, _c_; the skin of the back, _d_, is laid open, exposing the circulatory system; _e_, pharynx; _f_, intestine; _g_, muscles of the belly; _h_, gland, supposed to be the liver; _i_, generative organs; _j_, feet; _k k_, gills; _l_, heart; _m_, dorso-intestinal vessel; _n_, intestinal vessel; _n_, venous sinus; _o o_, ventral trunk, branching into smaller veins, _p_.] The slender filaments which radiate from the head of the tubicular worms are flattened, sometimes tortuous, always ciliated, and are often barred and variegated by bright purple, green, and yellow tints, forming a rich and gorgeous crown. The mucus, which cements together the particles of sand and shell for the artificial tubes of this kind of worms, is believed to be secreted from glands in the first segment of the body; but the long slender filaments of the head are the active agents in the structure. The tentacles are hollow bands with strong muscular edges, which the worm can bring together so as to form a cylinder, at any point of which it can take up a particle of sand, or a whole row of particles, and apply them to its glutinous body. The fibres at the free ends of the tentacles act both as muscular and suctorial organs; for when the worm is going to seize a particle of sand or food, the extremity of the tentacle is drawn in by the reflux of the colourless liquid in its interior, so that a cup-shaped cavity is formed in which the particle is secured by atmospheric pressure, aided by the power of the circular muscular fibres at the extremity of the tentacle. [Illustration: Fig. 134. Pushing poles of Serpula.]