3H ANIMAL BIOLOGY. [Part II. 



cut end examined with a lens, the anterior aorta will be seen 

 running along its dorsal surface. And if the rectum, a little 

 posterior to the ventricle, be similarly cut and examined, the 

 posterior aorta will be seen running beneath the rectum (along 

 its ventral surface). By the anterior aorta the blood is dis- 

 tributed to the foot, labial palps, intestine, and the anterior 

 part of the mantle. By the posterior aorta the adductor 

 muscles of this region and the posterior part of the mantle are 

 supplied. These vessels break up into irregular chambers or 

 lacunar spaces, whence the blood collects into a large vessel, the 

 so-called vena cava or central blood sinus (Fig. 91, A., c. b. s.), 

 which may readily be seen beneath the floor of the pericardial 

 chamber. Thence it passes by a number of vessels to an 

 irregular plexus in the renal organ ; and thence on each side to 

 a longitudinal vessel which lies in the gill-axis. From this 

 longitudinal vessel the blood passes into vertical vessels which 

 run downwards into the descending lamellae at the points of 

 union of the inter-lamellar junctions with the lamellar mem- 

 brane. There are similar vertical vessels in the ascending 

 lamellae \ but there does not seem to be a distinct capillary 

 system with definite walls between these two, but rather a 

 loosening or incoherence of the walls of the vessels, such as to 

 admit the passage of blood from them into the substance of the 

 lacunar tissue. The vertical vessels in the ascending lamellae 

 carry the blood upwards to horizontal vessels which run along 

 their margins, by which the purified and aerated blood is con- 

 veyed to the auricles. A plexus of vessels or spaces arises near 

 the junction of these afferent vessels with the auricles, and 

 ramifies in the so-called organ of Keber (Figs. 90, A, and 92, Jc. 0.), 

 which embraces the anterior end of the pericardial cavity. The 

 function of this organ is not well understood. 



Thus the general order and direction of circulation in the 

 mussel is : ventricle, aortae, smaller vessels and lacunar spaces 

 in the foot and mantle, central blood sinus, renal organ, gills, 

 auricles, ventricle. 



The blood is colourless, with numerous white amo3biform 

 nucleated corpuscles. 



