Passing now to the sedentary or fixed worms, we meet with some that are very interesting and beautiful creatures, even when considered apart from the wonderful homes they construct. The several species of the genus _Terebella_ form a soft and flexible tube by binding together particles of sand, shells, or mud with a sticky substance that exudes from their own bodies. These tubes are to be found in abundance between the tide-marks on almost every low, sandy shore, the nature of the tubes varying, of course, with the character of the materials at the disposal of the builder. In some cases the tubes are exposed throughout the greater part of their length, but very frequently they are more or less buried in the sand or other material of the beach, so that one has to dig to a moderate depth in order to extricate them. In either case, however, the tube of Terebella may be known by the free tufts of sandy threads that form a deep fringe around its mouth. These worms almost invariably select a sheltered situation for their abode, and should be searched for at the foot of rocks, or under stones, and it is no easy matter to move the buried tube with its occupant intact. When turning over the stones of a sandy or muddy beach one frequently discovers the slender, thread-like tentacles of the Terebella, together with the sandy filaments that surround the mouth of the tube, the remainder of the tube and its occupant being beneath the surface, and the ground is often so hard and stony that a strong tool is necessary to dig it out; but the work entailed will be amply repaid if a perfect specimen be obtained and placed for observation in the aquarium. [Illustration: FIG. 119.--TUBE-BUILDING WORMS: _Terebella_ (LEFT), _Serpula_ (MIDDLE), _Sabella_ (RIGHT)] The reader may possibly be acquainted with the tubes or cases that are constructed by the larvæ of caddis flies in fresh-water ponds and streams, and perhaps has noticed the ease with which these creatures may be made to construct new homes after having been turned out of doors. Similar experiments may be performed with Terebella; for when the worm has been extricated from its tube without injury--a work that requires great care on account of the soft and slender nature of the creature’s body--and placed in the aquarium with a bed of suitable material, it will build itself a new dwelling. As with the caddis larvæ, the different species may be known by the materials they select to construct their tubes, but in captivity they may be compelled to employ other than their favourite substance for this purpose. It is unfortunate, however, that Terebella is a nocturnal builder, and thus its movements are not so easily observed.