WE may frequently pick up on the seashore stones that are marked on the surface with a number of small, slit-like holes, as if they had been persistently stabbed with the sharp end of the blade of a penknife. Similar slits may also be found on the surface of the shells of oysters, mussels and
other shellfish. These are made by a small worm known as Polydora. On examination the slits prove to be in pairs and to be the openings of narrow tunnels which penetrate the rock and meet in the middle to form a U. In this retreat Polydora hides with its head protruding from one slit and its tail from the other; an admirable protection for such a soft-bodied animal. In addition, small chimney-like tubes of mud are often constructed over the slits to protect further the protruding ends of the body.
How such a hard material is bored by so small an animal is as yet uncertain, but it is possible that the skin gives off a fluid capable of dissolving the substance of the rock or shell. Among the oyster-beds considerable damage is done to the stocks by the boring habits of this marine tunneller, not because of actual injury to the oysters but because the shell is spoilt in appearance and the bivalve rendered less marketable. A Polydora living in the Red Sea, however, goes even further. This one enters the shell of an oyster, pushes its way between the soft body of the oyster and the inner surface of the shell, and irritates the oyster until it almost completely covers the worm with a layer of mother-of-pearl. Then, in the safe seclusion of the oyster’s shell, in a burrow obtained without labour or trouble, it feeds on the small animals living in the currents of water drawn in by the oyster’s gills.
For a striking contrast to the parasitic habits of the Red Sea Polydora we may turn to the evidence of commendable industry found on our own shores. Many worms build for themselves tubes of mud or sand-grains. Often these are of most delicate workmanship, the grains of mud or sand being carefully arranged in the walls of the tube and cemented together with as much precision as the bricks in the walls of human habitations. Some of these worms live singly, others in colonies, and on some parts of our coasts the colonial forms build reefs of sandy tubes several square yards in extent. Although the individual tubes are of such seeming delicacy, the reefs are capable of withstanding the battering of the surf four times a day as the tide rises and tails.