[Illustration: FIG. 135.--1. _Teredo navalis._ 2. _Teredo norvegica_]
Other ship worms belong to the genus _Teredo_, and are very similar in general characters. The shell is small and globular, with a wide gape at both ends, and consists of two three-lobed valves with concentric furrows. It is so small in proportion to the size of the animal that it encloses but a small portion of the body, and lies at the bottom of the burrow, which is of considerable length--often from one to two feet. The animal is very wormlike in form; and although the shell is so small, yet all the internal organs are enclosed by it. The mantle lobes are united in front, except where the sucker-like foot passes through them; the gills are long and narrow, and extend into the siphonal tube; and the two very long siphons are united almost throughout their length. It is also interesting to note that in these animals the rectum does not pass through the heart, as it does in nearly all molluscs, and that a pair of horny or calcareous 'styles' or 'pallets' project from the place where the two siphonal tubes begin to diverge.
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.