The mantle lobes of the animal are united only at a point between the two siphonal openings. There are two elongated gills on either side, and the foot is thick and more or less grooved. [Illustration: FIG. 149.--_Mytilus edulis_] Mussels inhabit salt, brackish, and fresh waters, generally attaching themselves by means of a silken byssus, but sometimes concealing themselves in ready-made holes, or in burrows of their own; and some even hide themselves in a nest which they prepare by binding together fragments of shells or sand. The edible mussel, which forms such an important article of diet, especially among the poorer classes in our large towns, may be easily distinguished from similar species of another genus by the very pointed umbones, and the coarse and strong fibrous byssus by which it clings to any solid object. It is found most abundantly on muddy coasts, and on mud banks in the estuaries of rivers, generally in such situations as are uncovered at low tide. The fry abound just below low-water level, and grow so rapidly that they reach their full size in a single year. It is well known that a diet of mussels occasionally produces very unpleasant and even dangerous symptoms in the consumer, and this result has been attributed to the action of a particular organ of the animal which has not been carefully removed before eating. This, however, is not the case, as proved by the fact that the eating of these edibles is usually perfectly safe when no such precautions have been taken. It is highly probable that the deleterious character referred to is due to a disease which sometimes attacks the mussels themselves, but the exact nature of this has not been thoroughly made out. [Illustration: FIG. 150.--1. _Modiola modiolus._ 2. _Modiola tulipa._ 3. _Crenella discors_] There is another genus (_Modiola_) containing several species commonly known as Horse Mussels, and these may be distinguished from _Mytilus_ by their habit of burrowing, or of constructing a nest by spinning together various fragments. The shell, also, is more oblong in form, and much swollen near the anterior end; and the umbones are not so pointed. The epidermis covering the shell is of fibrous structure, and often extends beyond the edges of the valves in the form of a fringe. Several species of Horse Mussels inhabit our shores, from low-water mark to a depth of fifty fathoms, but none of them is used for food. The commonest species is _Modiola modiolus_, which has a particularly strong byssus, and its fibres generally bind together such a number of stones &c. that the shell is completely hidden in the entangled mass. Other British species include _M. barbata_, so called on account of the peculiar fringed threads of the epidermis; _M. phaseolina_, in which the epidermis threads are not fringed; and _M. tulipa_, named from the streaks of crimson or purple that radiate from the umbones of the shell and remind us of the colouring of the tulip flower.