The Terebellids use their numerous tentacles in searching {290}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." [Illustration: 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. {291}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.