In front of the pectoral girdle lies a thin-walled sac, the _pericardial sac_. Open it by a median ventral incision. Remove about one-half inch of the middle of the pectoral girdle, being careful not to cut the thin-walled part of the heart lying dorsal to it. The _pericardial cavity_ is a pear-shaped chamber containing the heart, and lined by the smooth _pericardium_ which is morphologically equivalent to the peritoneum. At the anterior extremity of the chamber the pericardium is seen to be reflected backward over the surface of the heart, thus forming its smooth outer coat. The _heart_ may be considered as a bent tube, enlarged in certain regions to form the chambers. Anteriorly and ventrally is a short, thick-walled tube, the _conus arteriosus_; this leads out of the pericardial sac anteriorly, while posteriorly it opens into a large muscular chamber, the _ventricle_. Dorsal to the ventricle, and projecting on either side of it is the thin-walled _auricle_. Dorsal to both ventricle and auricle is the extremely thin-walled _sinus venosus_. This is triangular in shape, the apex opening into the posterior side of the auricle, the base attached to the posterior wall of the pericardial cavity; the lateral angles are drawn out into the _ducti cuvierii_, which receive veins from the anterior and posterior parts of the body. From the conus arteriosus springs a smaller vessel, the _ventral aorta_, which passes forward between the gill pouches. Take note of the small arteries passing over the surface of the conus and along the inner ends of the gill pouches, and take care not to cut them or their branches in the subsequent dissection. Two pairs of arteries leave the ventral aorta as it emerges from the pericardial sac. The aorta then passes forward some distance and finally divides into two branches which pass to either side. Follow the branches of the aorta outward on the left side and demonstrate their courses. The anterior branch quickly divides into two, the anterior of these passing along the base of the first demibranch. The posterior enters the septum between the first and second pouches, and supplies the second and third demibranchs. The middle branch of the aorta passes directly to the fourth and fifth demibranchs. The posterior branch divides almost as it leaves the aorta, its branches supplying the remaining demibranchs. There is considerable variation in this branch of the aorta. It usually divides as stated, but it frequently passes some distance toward the gills before dividing, and in a considerable number of cases two vessels arise directly from the aorta instead of one. The arteries carrying blood from the ventral aorta to the gills are named the _afferent branchial arteries_. Observe the relation of these vessels to the gills.