The Serpula and its allies are richly-coloured worms, living in contorted tubes with lids, frequently seen encrusting rocks, the shells of oysters, and other mollusca. By a peculiar mechanism of their bristly feet they can open the lid of their tube, push out their fan of gorgeous tentacles, pull it in again, and shut up the tube. As the protrusion of the worm from its tube is slow, cautious, and gradual, the retreat swift and sudden as lightning, there are two distinct sets of organs in the feet by which these motions are performed.[33] On the back of the worm there is a sort of shield, the sides of which bear seven pairs of wart-like feet, which are perforated for the working of protrusile microscopic bristles (fig. 134). Their upper parts are double-edged, with a groove between them, and serrated with close-set teeth. The organs of retreat are much more complicated and numerous. Mr. Gosse has computed that there are about 1,900 blades on the seven pairs of feet, each movable at the will of the worm, and that there are nearly 10,000 teeth hooked into the lining of the tube when it wishes to retreat. The manner in which it comes out of its tube and retires into it again is the same as that employed by the earth-worm. There are twenty-four genera of the order Errantia, or wandering sea-worms. Multitudes swarm on every coast; they have considerable muscular strength, and are highly irritable; some are called sea-centipedes, from the number of their feet and length of their segmented bodies, which are slender, and vary from a few inches or less to thirty-five or forty feet. They are generally coiled up under stones, or wander by the slipperiness of their smooth skins through masses of sea-weeds or shells at low tide. In most of them the rings are decidedly marked; the first and last segments are unlike, while the rest are mere repetitions one of another. Their locomotive organs are a pair of perforated fleshy warts on each of their numerous segments, through which groups of rigid, simple or barbed bristles are protruded and retracted.