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. 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. _Se_, palp; _P_, foot; _O_, mouth; _G_, liver; _Gg_, _Vg_, _Pg_, ganglia. ] The =food tube= of the mussel is comparatively simple. Behind the mouth it enlarges into a swelling called the _stomach_ (Fig. 193). The bile ducts of the neighbouring liver empty into the stomach. The _intestine_ makes several turns in the substance of the upper part of the foot and then passing upward, it runs approximately straight to the vent (or anus), which is in the wall of the exhalent siphon. The intestine not only runs through the _pericardial cavity_ (celome) surrounding the heart, but through the ventricle of the heart itself (Fig. 196). [Illustration: FIG. 196.—HEART OF MUSSEL, with intestine passing through it. ] The =kidneys= consist of tubes which open into the pericardial chamber above and into the gill chamber below (_Neph._, Fig. 193). The tubes are surrounded by numerous blood vessels (Fig. 198) and carry off the waste matter from the blood. [Illustration: FIG. 197. ]