MOLLUSCA. 115 



ventro-lateral wall of the pericardial cavity. An auricula-ventricular 

 valve is placed at the point of union of each auricle with the 

 ventricle. It is made up of two small flaps, which permit blood 

 to pass into but not out of the ventricle. 



The ventricle (through which the rectum runs) gives off two 

 arteries, an anterior aorta in front running forwards above the 

 rectum, and a posterior aorta behind running backwards below 

 the rectum. The anterior aorta divides into branches, which 

 supply the labial palps, the greater part of the alimentary canal, 

 the foot, the anterior adductor, and adjacent parts; while the 

 posterior aorta supplies the hinder part of the rectum, the posterior 

 adductors, and the greater part of the mantle. The ultimate 

 branches of these arteries end in minute irregular spaces (lacunae) 

 from which veins arise. These are vessels in which the blood is 

 flowing towards the heart. The foot and organs of the visceral 

 mass return their blood to the vena cava, a longitudinal vein lying 

 in the middle line immediately below the floor of the pericardial 

 chamber. The blood passes from the vena cava into the neph- 

 ridium on either side, where it enters a close network of channels. 

 From this network vessels run to the gills, opening on each side 

 into an afferent branchial vein, which runs along the junction of 

 the outer and inner gill-plate. Numerous branches pass into the 

 gill-substance from, this vein. The blood is returned to each 

 auricle by an efferent branchial vein, which takes a course along 

 the attachment of the outer lamella of the outer gill-plate on that 

 side. Efferent veins from the mantle-lobes also enter the efferent 

 branchial veins at each end. 



Course of the Circulation. The heart is systemic, receiving 

 pure blood from the gills and mantle-lobes, which passes into the 

 auricles, and thence into the ventricle. The former contract 

 together, filling the ventricle, which then also contracts in a 

 wave-like manner, and forces the blood through the arteries to 

 the body at large. From the smallest arterial branches the blood 

 passes into the lacunae, and thence into venous channels, which 

 are arranged as above described. The result is that the blood 

 from, the visceral mass and foot is purified in the nephridia, and 

 then in the gills, before reaching the auricles, while the blood 

 from the mantle-lobes is oxygenated there, and returned to the 

 auricles without passing through either nephridia or gills. 



4. Respiratory Organs (Figs. 32, 33, and 34). These consist 



