FRESHWATER MUSSELS 



sage from the point where the two inner sheets first unite, and 

 dorsal of their union, and will appear posteriorly at the exhalant 

 siphon. 



The " foot " is the large, orange, tongue-like organ lying 

 between the gills of the right and left sides. Its dorsal part is 

 softer than the ventral, and contains 

 the reproductive glands and large por- 

 tions of the digestive system. The 

 ventral part is, when contracted, hard 

 and very muscular. From the dorsal 

 posterior angle there passes back- 

 wards a strong thong of muscle, whose 

 fibres spread out fanwise over the 

 " foot/' to be inserted into the shell 

 and there cause the impression known 

 as that of the posterior " retractor 

 muscle of the foot." The other 

 muscles which find attachment to the 

 shell lie in the anterior dorsal region 



of the foot, and can be recognised by FlG . 35 ._ Diagram ofsec tion through 

 their fan-like radiating fibres ; their 

 ends, cut in opening the shell, are 

 visible behind the anterior adductor 

 muscle. The ventral, muscular portion 



of the foot is capable of being greatly distended, so as to pro- 

 ject like a blunt axe-head between and in front of the two valves. 

 The distension is brought about by the pressure of blood within 

 the " foot," there being a valvular arrangement whereby blood 

 is prevented from flowing out of it, in spite of the fact that the 

 heart continues to force blood into the pedal vessels and spaces. 

 Thus the whole organ becomes turgid and relatively firm, though 

 flexible. When crawling along the mussel slowly thrusts for- 

 ward the swollen " foot " into the mud, perhaps as much as 

 3 or 4 inches. There is thus made in the bottom a furrow 

 against whose sides a firm hold is obtained by mere pressure. 

 When the " foot " is fully extended its tip swells so as to press 

 harder against the sides of the furrow ; the valves are drawn 

 forcibly together so as to clip the upper part of the foot and 



mussel, a, outer gill ; b^ inner 

 gill ; c t shell ; d t mantle ; <?, foot 

 with cut ends of coils of intes- 

 tine. 



