5o6 THE INVERTEBRATA 



supplying the most anterior part of the body including the foot). The 

 arteries break up into a network of vessels in all the tissues and these 

 join to form veins and sinuses which are largely situated on the inner 

 side of the mantle and the superficial parts of the body. The skin, 

 being bathed in water and devoid of any cuticular covering which 

 might hinder diffusion, is a general organ of respiration and the mantle 

 is the most important part of it. Most of the blood from the^pallial 

 circulation is returned to the network of vessels in the kidney through 



Viscera 



A ^^-^ ^ B 



Ctenidia 



Fig. 372. Circulation of Anodonta. A, Simplified diagram to show the course 

 of the blood, indicating the relative importance of the various branches. 

 Vessels returning arterial blood to the heart shown in black. B, Transverse 

 section of Anodonta to show part of the course of the circulation. In the foot, 

 F., the veins run into the vena cava cut in section, from which a small part 

 of the blood is returned direct to the auricle in the dorsal wall of the bladder, 

 bl., the rest through the kidney, ^., longitudinal afferent vessels, Iv/, and thence 

 to the afferent system of vessels in the ctenidium, aff.c.v. On the other side 

 the efferent system of vessels, eff.c.v., is shown returning blood to the longi- 

 tudinal vessels at the base of the ctenidia, v.", from which it passes to the 

 auricle, au., through an irregular system of blood spaces. The pallial circula- 

 tion is not shown here. sbr. epibranchial space; ven. ventricle. 



the ribbon-like organs, known as plicate canals, which extend along 

 the mantle just above the insertion of the outer ctenidium. 



The visceral vessels likewise return blood to the kidney network 

 so that practically the whole of the blood passes through the excretory 

 organ and is purified. A part of the blood from the kidney network 

 enters the ctenidial circulation, discharging into the longitudinal 

 afferent branchial vein, which gives off to each filament a vessel which 



