The shells were held together by two large _adductor muscles_. The anterior adductor (Fig. 193) is near the front end, above the foot. The posterior adductor is toward the rear end, but not so near the end as the anterior. Can you find both _muscle scars_ in the shells? Are they nearer the ventral or dorsal surface? The points of attachment traveled downward and farther apart as the animal grew (see Fig. 191). Higher than the larger scars are small scars, or impressions, where the protractor and retractor muscles that extend and draw in the foot were attached. [Illustration: FIG. 193.--ANATOMY OF MUSSEL. (Beddard.)] The muscular _foot_ extends downward in the middle, halfway between the shells (Fig. 193). On each side of the foot and behind it hang down the two pairs of =gills=, the outer pair and the inner pair (Fig. 192). They may be compared to four V-shaped troughs with their sides full of holes. The water enters the troughs through the holes and overflows above. Is there a marked difference in the size of the two pairs of gills? A kind of chamber for the gills is made by the joining of the mantle flaps below, along the ventral line. The mantle edges are separated at two places, leaving openings called _exhalent_ and _inhalent siphons_. Fresh water with its oxygen, propelled by _cilia_ at the opening and on the gills, enters through the lower or inhalent siphon, passes between the gills, and goes to an upper passage, leaving the gill chamber by a slit which separates the gills from the foot. For this passage, see arrow (Fig. 194). The movement of the water is opposite to the way the arrow points. After going upward and backward, the water emerges by the exhalent siphon. The gills originally consisted of a great number of filaments. These are now united, but not completely so, and the gills still have a perforated or lattice structure. Thus they present a large surface for absorbing oxygen from the water. [Illustration: FIG. 194.--MUSSEL. _A_, left shell and mantle flap removed. _B_, section through body. =Question:= Guided by other figures, identify the parts to which lines are drawn.] The =mouth= is in front of the foot, between it and the anterior adductor muscle (Fig. 194). On each side of the mouth are the _labial palps_, which are lateral lips (Fig. 195). They have cilia which convey the food to the mouth after the inhalent siphon has sent food beyond the gill chamber and near to the mouth. Thus both food and oxygen enter at the inhalent siphon. The foot is in the position of a lower lip, and if regarded as a greatly extended lower lip, the animal may be said to have what is to us the absurd habit of using its lower lip as a foot. The foot is sometimes said to be hatchet-shaped (Fig. 195). Do you see any resemblance? Does the foot penetrate deep or shallow into the sand? (Fig. 190.) Why, or why not? [Illustration: FIG. 195.--MUSSEL. From below. Level cut across both shells.