The cavity thus exposed is the _pericardium_. Note within the pericardium a long tube extending through it. This is a portion of the alimentary canal, the _rectum_, which opens posteriorly through the _anus_ into the _supra-branchial chamber_. Note a muscular sac about the rectum midway of its course through the pericardium. This is the unpaired _ventricle_ of the _heart_. Attached to each side of the ventricle are thin-walled sacs, the _right_ and _left auricles_, which are entered by fine blood-vessels, the _efferent branchial veins_, from the right and left gills. The blood brought through these blood-vessels from the gills flows into the auricles and from them into the unpaired muscular ventricle, from which it is forced anteriorly and posteriorly through two main arteries, the _anterior_ and _posterior aortas_, to all parts of the body. After bathing the body-tissues the blood is collected into a median longitudinal vein beneath the pericardium called the _vena cava_. From the vena cava the blood passes through the kidneys and gills to be returned at last to the heart. The mantle acts as an organ for the aeration of the blood, and the blood it receives or at least part of it passes directly back to the heart without passing through the kidneys and gills. Note the delicate membranous dark-colored sac on the floor of the pericardium, the _kidneys_ or _nephridia_. These are paired structures which appear as two U-shaped tubes lying side by side. Each consists of a lower portion with thick folded walls, the kidney proper, and an upper thin-walled portion, the _ureter_. The kidneys open internally through a pair of _reno-pericardial_ openings into the pericardium, while the ureters communicate with the mantle-cavity by an opening on the side of the body beneath the gills as already mentioned. The kidneys are profusely supplied with fine blood-vessels and carry off the waste matter from the blood. Beneath the posterior adductor muscles note a small white spider-shaped body, the more or less united _visceral ganglia_ of the nervous system. Posteriorly these ganglia give off nerves to the mantle and gills, while anteriorly there proceed two nerves, the _cerebro-visceral connectives_, running forward, one on either side of the foot close to the visceral mass, to the _cerebro-pleural ganglia_, paired ganglia lying near the mouth. A delicate commissure running over the gullet connects these ganglia. TECHNICAL NOTE.--Cut away the skin and outer muscular layer from the left side of the foot.