_Se_, palp; _P_, foot; _O_, mouth; _G_, liver; _Gg_, _Vg_, _Pg_, ganglia.] [Illustration: FIG. 196.--HEART OF MUSSEL, with intestine passing through it.] [Illustration: FIG. 197.] 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 neighboring 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). 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. The =nervous system= consists of _three pairs of ganglia_ and nerves (Fig. 197). The ganglia are distinguishable because of their orange color. The pedal ganglia on the front of the foot are easily seen also; the visceral ganglia on the posterior adductor muscle may be seen without removing the mussel from the shell (Fig. 193). The reproductive organs open into the rear portion of the gill cavity (Fig. 193). The sperms, having been set free in the water, are drawn into the ova by the same current that brings the food. The eggs are hatched in the gills. After a while the young mussels go out through the siphon. =Summary.=--In the gills (Fig. 198) the blood gains what? Loses what? From the digestive tube the blood absorbs nourishment. In the kidneys the blood is partly purified by the loss of nitrogenous waste. [Illustration: FIG. 198.--DIAGRAM OF MUSSEL CUT ACROSS, showing mantle, _ma_; gills, _kie_; foot, _f_; heart, _h_; intestine, _ed_.] The cilia of the fringes on the inhalent, or lower, siphon, vibrate continually and drive water and food particles into the mouth cavity. Food particles that are brought near the labial palps are conveyed by them to the mouth. As the water passes along the perforated gills, its oxygen is absorbed; the mantle also absorbs oxygen from the water as it passes. The water, as stated before, goes next through a passage between the foot and palp into the cavity above the gills and on out through the exhalent siphon. By stirring the water, or placing a drop of ink near the siphons of a mussel kept in a tub, the direction of its flow may be seen. The pulsations of the heart are plainly visible in a living mollusk.