Again, we shall find that some of our live bivalves have protruded a thick, conical, fleshy mass--the _foot_, from the opposite end of the body. This organ is the means of locomotion in the case of the burrowing and other free-moving bivalves, but is developed to a less extent in those species that lead a sedentary life. Thus, the common Edible Mussel secretes a tuft of strong silky fibres (_byssus_) by means of which it fixes itself to a rock or other body, and therefore does not need the assistance of a muscular foot; and an examination of its body will show that the foot is very small in proportion to the size of the animal, as compared with that of the wandering and burrowing species. The same is true of the oyster, which lies fixed on its side, the lower valve being attached to the surface on which it rests. [Illustration: FIG. 130.--_Mytilus edulis_, WITH BYSSUS] We have made use of the terms _dorsal_ and _ventral_ in speaking of the shell of a bivalve, and it is important that these and a few other similar terms be well understood by those who are about to read the descriptions of the animals, or who may desire to describe them themselves. To do this, take a bivalve in your hand, and hold it before you in such a position that the hinge is uppermost, and the siphons turned towards you. The foot of the animal is now pointing in the direction you are looking, and the mouth, situated at the base of the foot, is also directed the same way. You have now placed the shell, and, of course, also the animal, in such a position that its _dorsal_ side is uppermost, the _ventral_ side below, the _anterior_ end turned from you, the _posterior_ (often narrower) end towards you, the _right valve_ on your right, and the _left valve_ on your left. Knowing the exact uses of these few terms you are in a better position to understand the descriptions of bivalves, and to locate the exact situations of the various internal organs named in such descriptions. A great deal of the internal anatomy of a bivalve mollusc may be made out by easy dissections, and although the structure of the different species varies in several details, the general characteristics of the group are practically the same in all and may be gathered by the examination of a few specimens. [Illustration: FIG. 131.--A BIVALVE SHELL (_Tapes virgineana_) _a_, anterior; _p_, posterior; _l_, left valve; _r_, right valve; _u_, umbo, on dorsal side]