Page:The common shells of the sea-shore (IA commonshellsofse00wood 0).pdf/19

Rh the upper aperture affording a passage for the two siphons, or tubes, and the lower being intended for the muscular "foot" which projects between the shells, and by pressure against the wall of the burrow, forces the shell to perform the peculiar movements which insinuate it into the substance into which it is imbedded.

In the accompanying illustration, fig. 1, is shown the inner surface of the Piddock shell, in order to show its general structure and the points which are particularly worthy of notice. Towards the side of the shell are the “scars” which represent the marks of the muscles which draw the shell together; while the waved line towards the base of the shell shows the attachment of the mantle. This line will be hereafter described. If the shell be procured in a perfect state, some remarkable peculiarities of structure are to be seen.

Towards the base, and on the back, is a thickened portion of the shell, of a bridge-like form, and having a chambered structure, as is seen at fig. 3. There