//. ACEPHALA. 



363 



{Aeephala). Hence there are only two divisions in the body, 

 dorsally the visceral sac and ventrally the foot. The foot, degener- 

 ate in many, has a broad sole only in Pectunculus and the Nuculi- 

 dre; usually it is hatchet-shaped (Pelecypoda), that is, compressed 

 with a rounded ventral margin. It may be enormously expanded 

 and contracted again. This expansion is often explained by the 

 taking of water into the blood, but now it is generally accepted 

 that it is accomplished by forcing blood from other regions into 

 it. While the foot by this extensibility can serve as a locomotor 

 organ, it also functions in many as an organ of attachment. 

 Inside is a large byssus gland which can secrete silky threads, the 

 fcyssus (fig. 353), one end of which is fastened to foreign objects by 



FIG. 353. Mytilus edulis*. (After Blanchard.) a, edge of mantle ; b, spinning finger of 

 foot ; c, byssus ; d, e, retractors of foot ; /, mouth ; 0, labial palpi ; 7t, mantle ; i, j, 

 inner and outer gills. 



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

 remains in connection with the foot. Molluscs which have a byssal 

 gland are found anchored by a thick bunch of 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 auricles (figs. 351, 352, 7* 1 , 7* 2 ). The auricles receive the 

 blood direct from the gills; the ventricle forces it out through 

 anterior and posterior aortae (fig. 351), the latter lacking in many 

 species. 



The excretory organs (organs of Bojanus) lie immediately 

 below the pericardium. The organs of the two sides touch in the 



