Several species of _Teredo_ are to be met with on our coasts, but they are so similar in general structure that the above brief description applies almost equally well to all. Other boring molluscs frequent the British shores, but they belong to quite a distinct family called the _Gastrochænidæ_ because their shells gape widely on the ventral side. Their valves are equal in size and very thin, the hinge has no teeth and the pallial line is sinuated. The margins of the mantle lobes are thickened and united except where a small aperture is left for the protrusion of the finger-like foot. The siphons are very long and retractile, and the gills extend into the inhalent tube. These animals burrow into mud, shells, or stone, often dwelling together in such numbers that their galleries cross one another and form a most intricate network, and the different species are to be found from low-water mark to a depth of a hundred fathoms or more. [Illustration: FIG. 136.--_Gastrochæna modiolina_ 1, Animal in shell; 2, shell; 3, cell] The British species belong to two genera--the typical genus _Gastrochæna_, and the _Saxicava_ or stone-borers. The former contains the Common Flask shell (_G. modiolina_) which burrows into limestone and shells, in the latter case passing generally through the shells into the ground below, and completing its home by cementing together any fragments of hard material that come in its way into a flask-shaped cell. The opening of the burrow is shaped like an hour-glass, the two expansions serving for the protrusion of the siphonal tubes, and the neck of the flask-shaped abode is usually lined with a calcareous layer that projects slightly to afford further protection to the extended siphons. Although this species is very common on some parts of our coast, it is seldom obtained without the aid of a dredge, for it usually lives at a depth of from five to ten fathoms; and when found it is generally no easy matter to extricate them from their holes, to the sides of which they often cement their shells. The genus _Saxicava_ contains a few species that drill holes, often several inches deep, in shells and stone, and frequently do great damage to breakwaters and other artificial structures. The foot is usually provided with a byssus by which the animal fixes itself to a little projection on the side of its burrow. The species are to be found from low-water mark to a depth of one hundred fathoms or more. The next family, named _Anatinidæ_, contains a number of molluscs that burrow in mud or sand or live in seclusion in the crevices of rocks. Their shells are thin, with a granulated outer surface, and the valves are united by a thin external ligament. The inner surface is pearly, the pallial line usually sinuated, and both valves are pitted for the reception of the somewhat stout internal cartilage. The mantle lobes are united, as are also the siphons to a greater or lesser extent; and there is only one gill on each side.