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.
'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.
'When the tentacle is about to seize an object, the extremity is drawn in, in consequence of the sudden reflux of fluid in the hollow interior. By this movement a cup-shaped cavity is formed, in which the object is securely held by atmospheric pressure. This power is, however, immediately aided by the contraction of the circular muscular fibres. Such, then, are the marvellous instruments by which these peaceful worms construct their habitation, and probably sweep their vicinity for food.'
The foregoing beautifully and accurately describes the tentacular cirri. The use of these organs in the formation of the tube in which the Annelid dwells, I will now endeavour to make clear, from close personal observation.
It is an extremely interesting sight to watch a Terebella extend its tentacles in all directions in search of building materials, catching up the surrounding molecules (sometimes visible along the whole length of each filament), and then, by a strong muscular contraction, bringing the collected atoms to the opening of the tube, around which, as is generally supposed, they are then immediately attached by a secretion which is exuded from the body of the animal. Such, however, is not the case.
When the filaments bring their 'subscriptions,' the material thus gathered, instead of being used at once for building purposes, is, in reality, first eaten by the animal, and, after undergoing a kind of mastication, becomes coated with a salivary secretion, and is then ejected in mouthfuls at the extremity of the tube, which, by such means, becomes gradually elongated. The shell work, when deposited as above mentioned, is held in position, and prevented from falling over the outside of the cavity, by the filaments which are made to hang down in a most ingenious manner; the animal, at the same time, putting itself in motion, allows the slimy surface of its body to press and rub against the new addition to the tube, which is thus effectually strengthened and soldered together.
The animal does not always wait until the opening of the tube is reached, but gently disgorging while lying at its ease, it then pushes forward by aid of its head and tentacles the mass of building material, which soon becomes distributed and moulded to its proper shape.
If your specimen should happen to build a complete tube, its mode of working cannot be well seen; but should it economize its labours, and run its house up in a semi-circular form against the transparent side of the vessel, as the animal I write of did, you will be enabled to see distinctly every movement that goes on in the interior.
It seems somewhat singular that the Terebella should possess the power of turning itself within its tube, so as to be able to extend its habitation from either end at will. I have very often watched the operation with emotions of pleasure, not unmixed with wonder.
Wishing to test the powers and intelligence of my specimen, I dropped within its tube, which was curved in shape lengthways, some particles of sand, and a pebble which nearly filled up the 'bore' of the cylinder. The great annoyance occasioned by this intrusion to the master of the house was painfully evident. For a whole day the Terebella endeavoured to push out the objectionable matter by means of its head and cephalic tentacula, but without success; for although the mass frequently neared, it never touched or toppled over the mouth of the aperture, and consequently fell to its original position at the bottom of the tube as soon as the animal removed the pressure.