After the lapse of a few days the building operation was resumed, and the tube carried fully an inch further, at right angles to its former position. The opposite end of the structure was next extended at an angle of 45° from the base of the vase, to a height of about two inches. Then commenced a very curious phenomenon. Some of the tentacles were incessantly elevated and extended across the vessel, until they touched the opposite end of the tube, with what object I could not then conceive. The design, however, was afterwards made evident: in less than two days the animal succeeded _in making both ends of its tube meet together, so as to form a continuous circle_. I happened to be watching the mason, when the last stroke of his labial trowel was given to the building, and shall never forget the cautious way in which the animal crept for the first time over the newly-completed portion of the work, and the seeming delight with which it continued to glide, hour after hour, over the entire circuit of its dwelling-place. Sometimes its body would be long drawn out, until the tips of the tentacula would reach, and apparently tickle the extreme point of the tail; then a race would commence, in character exactly resembling that so often witnessed with the kitten, or the playful whelp, when either of these animals foolishly imagine that the tip of their tail is adorned with some coveted tit-bit. The branchial organs of my specimen were very beautiful objects, being formed of three blood-red spiral tufts, the effect of which were heightened by their being placed in contact with the drab, coloured cephalic[10] tentacles, which seemed to be almost innumerable. These latter organs, although apparently so useless when seen closed, are in reality of the greatest importance to the _Terebella_, for they not only act as auxiliary organs of respiration, and aid most materially in building its dwelling-place, but also constitute the real organs of locomotion. [10] Cephalic, belonging to the head. 'They consist,' says Dr. Williams, 'of hollow flattened, tubular filaments furnished with strong muscular parietes. The band may be rolled longitudinally into a cylindrical form, so as to enclose a hollow cylindrical space, if the two edges of the band meet, or a semi-cylindrical space if they only meet imperfectly. This inimitable mechanism enables each filament to take up and firmly grasp _at any point of its length_ a molecule of sand, or, if placed in a linear series, a row of molecules. But so perfect is the disposition of the muscular fibres at the extreme free end of each filament, that it is gifted with the twofold power of acting on the sucking and muscular principle.