II. ACEPHALA 



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 bysstis gland which can secrete silky 

 threads, the byssus (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 



FlG. 



Fig. 



323. no. 324. 



Fig. 323. — Projection of section through foot and heart of lig. 322. h\ &^, upper 

 and lower limbs of nephridium; d, intestine; c, nephridiopore; /«, foot; g, gonad; 

 A', ventricle surrounding the intestine; h', auricle; /;■', k-, inner and outer gill lamellae; 

 I, hinge hgament; m, mantle; n, cerebro-visceral commissure; sp, nephrostome; v, 

 venous sinus. 



Fig. 324. — Mytilus edulis* (after Blanchard). a, edge of mantle; J, spinning 

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

 i, j, inner and outer gills. 



auricles (figs. 322, a, v, 323, /z', Jr). The auricles receive the Ijlood direct 

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

 aortffi (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, Ijoth connected behind 

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

 21 



