Fig. 154.—Terebella conchilega Pall. Upper end of the tube (s) showing the anterior end of the worm. h, Its head; t, tentacles collecting sand grains (y) in their grooves; x, sand grains in mouth of worm; f, filamentous fringe of tube. (After Watson.)
The Terebellids use their numerous tentacles in searching for particles of sand, etc.; each tentacle is grooved along its ventral surface, and the particle is conveyed along the furrow to the mouth. These particles are actually taken into the mouth, and mixed with some sort of secretion; on ejection again, each particle is placed by another tentacle in its position at the edge of the tube, and by means of its lower lip the Terebellid works it into place.[[347]]
But whereas the greater number of tubicolous worms make use of adventitious material wherewith to strengthen the wall of their tube, the Serpulidae secrete carbonate of lime from their tube-glands, and mould a tube of this substance. Amongst the Eunicidae the secreted substance is of itself strong enough to protect the animal; for in Hyalinoecia and species of Eunice the tube consists of a translucent, tough, parchment-like material.
Chemical analysis has been employed in a few cases to determine the substance composing the tube. In the case of Hyalinoecia (sometimes erroneously called Onuphis) the material consists of a phosphoric salt containing magnesia and a characteristic organic substance "onuphin"[[348]]; in Spirographis, a Sabellid, the name "spirographin" is given to its special secretion, whilst in Serpulids the organic base of the calcareous tube is "conchiolin."
Fig. 155.—Eunice tibiana Pourt. × ½. The branching tube (t) with the worm (w) protruding its head through one of several openings. (From Ehlers.)
The majority of worms are solitary, but there are a few instances of social worms—not that there is any co-operation or distribution of labour amongst the individuals, but they merely occur together in quantities; thus the sandy tubes of Sabellaria may form compact masses of several cubic feet, which, left uncovered by the receding tide, look like rocks upon the shore; as, for instance, at Paignton and Torquay. Filigrana implexa and Serpula uncinata similarly intertwine their calcareous tubes to form masses.
Whereas most worms live at the bottom of the sea, at various depths, a few are to be found at the surface. Purely pelagic habits are confined to a few families, viz. Tomopteridae, Typhloscolecidae, and the Alciopids and others amongst the Phyllodocidae; though Nectochaeta, one of the Polynoidae, and Ophryotrocha, one of the Eunicidae, are modified for this mode of life.[[349]] Several genera become pelagic during the breeding season. All these forms are excellent swimmers, and many of them are transparent.
The Colouring of Polychaetes.—The majority of Polychaetes quickly lose their colour in spirits, and become uniformly dull or light brown in museums. There are a few, however, which retain their brilliancy, like Aphrodite and Chloeia, but in both cases the coloration is due to the beautiful hair-like bristles ranged along each side of the animal; in the former the colours of the rainbow flash from specimens which have been kept in spirit for any length of time. The Polynoids, too, with their golden chaetae and pigmented scales, retain to some extent their characteristic colouring. But the colours of most Annelids are due to pigments in the skin, together with the haemoglobin of the blood, which are soluble, or otherwise changed, in alcohol; for instance, the bright greenish-blue tint of the common Phyllodoce of our coasts is changed to a rich chocolate brown; but such cases are rare, most worms becoming more or less decolorised.