II. ACKFHALA 



321 



plished by forcing blood from other regions into the foot. While this 

 makes the foot on organ of locomotion, it often serves as a means of 

 attachment. Inside is a large byssiis gland which can secrete silky 

 threads, the byss-ns (fig. 324), one end of which is fastened to foreign 

 objects by means of a finger-like process of the foot, while the other end 

 remains in connection with the foot. Molluscs with a byssal gland are 

 found anchored by byssal threads to stones, etc. 



The heart, surrounded by a pericardium, usually occupies the most 

 dorsal part of the visceral sac. It consists of a ventricle and a pair of 



FIG. 323. FIG. 324. 



FIG. 323. Projection of section through foot and heart of fig. 322. ?', b-, upper 

 and lower limbs of nephridium; </, intestine; e, nephridiopore; fu, foot; g, gonad; 

 h 1 , ventricle surrounding the intestine; lr, auricle; &', k-, inner and outer gill lamellae; 

 I, hinge ligament; m, mantle; n, cerebro-visceral commissure; sp, nephrostome; r, 

 venous sinus. 



FIG. 324. Mytihts edulis* (after Blanchard). a, edge of mantle; b, spinning 

 finger of foot; c, byssus; </, e, retractors of foot;/, mouth; g, labial palpi; h, mantle; 

 i, j, inner and outer gills. 



auricles (figs. 322, a, v, 323, Ii l , /r). The auricles receive the blood direct 

 from the gills; the ventricle forces it out through anterior and posterior 

 aortae (fig. 322), the latter lacking in many species. The excretory 

 organs (organs of Bojanus) lie immediately below the pericardium (fig 

 322, n}. Each consists in fresh water mussels of a dorsal smooth-walled 

 chamber and a lower portion traversed by threads, both connected behind 

 but separated elsewhere by a thin partition. The lower chamber is con- 

 21 



