Page:The common shells of the sea-shore (IA commonshellsofse00wood 0).pdf/26

16 rical, and covered with cilia or hair-like projections, by means of which it swims rapidly through the water. In thirty-six hours it assumes a new form, and speedily changes it for another, after which it returns again to its original form, so that in a very few hours the little creature is first spherical, then oval, then triangular, and then spherical again. In this stage of existence it possesses a foot, which enables it to crawl, after the manner of snails, and also has organs of hearing and sight.

It does not enjoy its locomotive powers for any long time, but fixes itself to some suitable object, passes through its last change, becomes a veritable shipworm, and begins its life-long task of boring. Opinions have long been divided on one point, namely, whether or not the ship-worm eats the material in which it bores. Many arguments have been used on both sides of the question ; but there is now little doubt that the animal obtains the whole of its sustenance from the water which is perpetually driven through its body, and that it gains little or no nourishment from the wood into which it happens to burrow.

At fig. 7 is shown a remarkable example of a ship-worm's tube. In pushing forward its burrow, the ship-worm has accidentally cut its way into a hole whence a bolt has been extracted; and instead of merely filling up the aperture, it has extended its tube into the bolt-hole, producing the singular modification which is represented in the figure. This remarkable specimen is in the collection of Mr. G. B. Sowerby.

At page 14, figs. 8 and 9, are shown two specimens of an allied shell, called the (Xylophaga dorsalis). Fig. 8 exhibits the closed shell, with the pair of accessory valves over the hinge; and fig. 9 depicts the living creature buried in wood, to show the manner in which it bores across the grain.