The =nervous system= consists of _three pairs of ganglia_ and nerves (Fig. 197). The ganglia are distinguishable because of their orange colour. 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 the 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 mollusc. =Habits of the Mussel.=—Is it abundant in clear or in muddy water; swift, still, or slightly moving water? Describe its track or furrow. What is its rate of travel? Can you distinguish the spots where the foot was attached to the ground? How long is one “step” compared to the length of the shell? The animal usually has the valves opened that it may breathe and eat. The hinge ligament acts like the case spring of a watch, and holds the valves open unless the adductor muscles draw them together (Fig. 189). [Illustration: FIG. 199.—OYSTER. _C_, mouth; _a_, vent; _g_, _g′_, ganglia; _mt_, mantle; _b_, gill. ] When the mussel first hatches from the egg, it has a triangular shell. It soon attaches itself to some fish and thus travels about. After two months it drops to the bottom again. [Illustration: FIG. 200.—TROCHUS. ]