British Razor-shell (_Solen siliqua_). _a_, foot; _b_, mantle; _c_, inhalant siphon; _d_, exhalant siphon; _e_, shell. ] [Sidenote: Cases 199–201.] Many of the _Solenidæ_, or Razor-shells, possess very elongated shells, and are remarkable for the great development of the foot, which can be pointed or contracted as may be required for boring into sand. By means of this powerful foot the animals, when disturbed, bore with such rapidity and to such a depth that their capture is a matter of great difficulty; and even when seized they hold on so tightly that at times they suffer their foot to be torn off rather than be captured. They not only burrow in sand, but also have the power of darting through the water, like the Scallops. Solens were considered a dainty dish by the ancient Greeks, and numbers are still eaten by the poorer coast-population of this country and abroad. [Sidenote: Cases 201–202.] The _Pholadidæ_, or Piddocks, are very remarkable shells, of an unusually complicated structure, some having the power of boring into rocks, wood, mud, sand, etc. Their shells are white, adorned with prickly sculpture, and, although thin, are strong. The foot is believed to be the principal excavating instrument, but the shell no doubt is used as a file to enlarge the hole as the creature grows. These animals are brightly phosphorescent; and certain species are eaten at many places on the shores of the Mediterranean. They appear to be indifferent as regards the material they bore into; for the common _Pholas dactylus_ (Fig. 36) of our own shores has been found in slate-rocks, mica-schist, coal-shale, new red sandstone, chalk, marl, peat, and submarine wood. The siphons are long in the Piddocks, united except near the end, and enclosed in tough skin. The species are world-wide in their distribution, and several are found fossil in some of the Tertiary formations. [Illustration: Fig. 36. Piddock, or Borer (_Pholas dactylus_). (From the British coast.) 1. Animal in the shell: _a_, foot; _b_, siphons; _c_, inhalant orifice; _d_, exhalant orifice. 2. Shell: _e_, accessory valves or plates. ] [Sidenote: Case 202.]