Page:The common shells of the sea-shore (IA commonshellsofse00wood 0).pdf/23

Rh siphons. The next point to be noticed is the extremity of the siphons, which are surrounded with a radiating appendage, something like the disc of a daisy, dandelion, or other composite flower.

The shell is peculiarly fragile, and so delicate, indeed, that it seems incapable of boring into the hard substances through which it makes its way. A specimen now before me is certainly not thicker than writing-paper, and scarcely so thick as that which is considered of the best quality. The cup still adheres to the end of the shell, which is deeply imbedded in hardened red clay. Removing the shell is a really difficult task, even for a practised hand, so delicate is its structure. It is a small species, rarely exceeding an inch and a half in length.

It is rather a remarkable fact that the shape of the shell alters considerably with age. When very young, the peculiar cup is not visible, and other portions of the shell are not fully formed. The reader will probably have remarked that the two siphon tubes are not separate, as is the case with the Piddocks, but that they are fused together, as it were, into a single tube, at the end of which the two orifices are situated.

To the same family as the Piddocks belong those wonderful and destructive molluscs popularly called Ship-worms. In spite, however, of their popular name, they are not worms at all, but are true shell-bearing molluscs, the shell being so small that for a long time it was mistaken for the jaws of the supposed worm.

At Plate I., fig. 13, is seen the typical species, Teredo navalis, as it appears while lying in its burrow, which is always made in wood. The shelly valves are seen at the bottom, while at the top the two siphons project. If the reader will compare this figure with that of the Piddock at fig. 12, he will at once see the resemblance between them, and that although in the Ship-worm the shell is very short and the body very