Living mussels which have been placed in a dish with mud several inches deep and covered with water will be seen to travel in a definite direction. The end which is in front is the head end. Note the process of thrusting out and retracting the fleshy _foot_ which extends between the two _valves_ of the _shell_. Note that the two valves are held together along the upper, or dorsal, surface by a horny structure, the _hinge-ligament_. Note near the hinge-line a prominence (_umbo_) in each valve from which extends a series of concentric lines of growth. The umbo is the oldest part of the valve. Note at the lower edge of the valves a soft membrane with a fringe along its free border. This is the edge of the _mantle-lobes_, flaps of the body-wall which cover the body and which aid in the functions of respiration and nutrition. TECHNICAL NOTE.--Specimens which are to be dissected should be killed by dropping them for a few seconds into warm water, when the muscles will relax enough so that a chip may be thrust between the valves. If specimens are to be kept for some time before dissecting they should be preserved in alcohol or 4% formalin. In a dead specimen carefully remove the left valve. This is accomplished by slipping in a thin knife-blade close to the inner edge of the left valve and carefully cutting the two large adductor muscles which bind the valves together. The dissection should be made under water. Before the removal of the valve, as just described, notice a portion of the mantle adhering to the inner face of the valve, along a line of attachment indicated by a crease. This is the _pallial line_. After the left valve has been removed, the mantle being carefully separated from it, note the large conical projections from the valves, the _hinge teeth_, which fit into each other. Note the large muscle impression just in front of the hinge-teeth; this is the point of attachment of the _anterior adductor muscle_, while just behind and adjoining it is the impression of the _anterior retractor muscle_. Note posterior to the adductor and below the retractor a small impression which affords attachment for the _protractor muscles_ of the foot. At the other end of the valve, note the large impression of the _posterior adductor muscle_ with the impression of the small _posterior retractor muscle_ just above it. TECHNICAL NOTE.--Lift back the left mantle-lobe, thus exposing the body parts underneath.