MOLL USKS 



103 



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 nourish- 

 ment. In the kidneys the blood is partly purified by the loss of 

 nitrogenous waste. 



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 ex- 

 halent siphon. By stirring the water, 

 or placing a drop of ink near the 

 siphons of a mussel kept in a tub, FIG. 

 the direction of its flow may be seen. 

 The pulsations of the heart are 

 plainly visible in a living mollusk. 



Habits of the Mussel. Is it abundant in clear or 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 un- 

 less the adductor muscles draw them together (Fig. 189). 



198. DIAGRAM OF 

 MUSSEL CUT ACROSS, 

 showing mantle, ma ; gills, 

 kie ; foot, /; heart, h ; in- 

 testine, ed. 



