The tentacles are employed in two distinct ways:—They may be flattened into slender ribbon-like structures, which, by being folded longitudinally at any point, may be made to grasp a particle of sand; and, in addition to this, the tip of the tentacle may be converted into a minute cup-shaped sucker by the withdrawal of the fluid it contains into the body.
Some species of Terebella build their tubes of ordinary sand, while others select fragments of shells. Some employ mud only, and occasionally we meet with tubes constructed of the silky secretion of the body with hardly any foreign matter.
We sometimes see edges of rocks, on low, sandy shores, covered with what appears to be large masses of consolidated sand, full of holes a little more than an eighth of an inch in diameter; and these masses are often so extensive and so firm that they seem to form the greater part of the rock itself. Such masses are particularly abundant on the south coasts of Devon and Cornwall, but are more or less plentiful on most sandy shores of Great Britain. They consist of the tubes of a species of the marine worm Sabella, which have been built up much in the same manner as those of Terebella, but usually exist in such numbers in the same spot that, together with the sand that has been washed between them, they form the dense masses just described.
A cluster of some dozens of these tubes may be detached with the aid of a hammer and chisel; or, in some instances, where the mass of tubes is not held so firmly together, by the mere pressure of the hand; and it will then be observed that each tube consists of a flexible membrane, of a somewhat leathery nature, formed by a sticky secretion from the body of the worm, with its outer surface covered with grains of sand. The tubes may be easily opened, and the occupants extracted for examination, when it will be observed that the front or upper portion of the worm is short and thick, while the hindmost portion is much thinner, and is doubled forwards in the tube. The body is also provided with numerous bristles, by means of which the worm is enabled to grasp the membranous lining of the tube, and thus secure a firm hold within its home.
A cluster of these tubes should be placed in a rock pool, or in the marine aquarium, when the worms may be seen to protrude gradually, and expose a large number of feathered tentacles, which, by their incessant motion, keep up the constant circulation of the water for the purpose of respiration as well as to bring food particles towards the mouths of the worms.
It is possible to keep these worms alive for some time in the aquarium, but special care is required for the reason that it is a very difficult matter to secure a cluster of tubes without injury to a certain number which are sure to be broken or otherwise damaged; and these, dying and decomposing within their homes, speedily pollute the water. Hence it is necessary to keep a sharp watch for dead specimens, which should, of course, be removed at once. The presence of a putrefying worm may often be detected by the appearance of a whitish fungoid growth round the mouth of what appears to be an empty tube; and if, through neglect, the water of the aquarium has been allowed to become contaminated by the products of decomposition, it will often happen that some of the living worms will come entirely out from their tubes, as if to seek a more sanitary situation. Thus, the exit of worms from their homes may always be looked upon as pointing to a suspicious condition of the water which, if not corrected immediately, may lead to the death of all.
The species we have briefly described is by far the commonest of the genus Sabella, but there are several others to be found on our shores. Some are of a solitary nature, and construct a sandy tube so much like that of a certain species of Terebella that they may be mistaken for that genus. Another solitary species builds a hard stony tube of carbonate of lime that has been extracted from the sea water; and although it is hardly possible to take the live worm from this calcareous tube without injury, the animal may be obtained intact for examination or preservation by dissolving away the tube in dilute hydrochloric acid.
Fig. 121.—A tube of Serpula attached to a Shell