At the anterior end of the body in front of the liver and between the sets of gills note the small _pericardial cavity_ within which is contained the _heart_. The pericardial cavity is separated from the peritoneal cavity by a thick muscular wall against which the liver abuts. The heart consists of four parts. The posterior part is a thin-walled reservoir, the _sinus venosus_, into which blood enters through the _jugular vein_ from the head and through the _cardinal vein_ from the kidney. From the sinus venosus it passes forward into a large chamber, the _auricle_. Next it flows into the _ventricle_, where, by the contraction of the walls, rhythmical pulsations force it into the _conus arteriosus_, thence into the _ventral aorta_, and lastly into the gills, where it is purified. After passing through the capillaries in the fine gill-filaments it is again collected, now pure, by paired arteries from each pair of gills, which arteries unite to form the _dorsal aorta_ extending backward just below the backbone to the end of the tail. From the dorsal aorta a pair of arteries, the _subclavian_, are given off to the pectoral fins. At this point two other arteries branch off ventrally, the first being the _cardiac artery_, which distributes blood to the stomach and pyloric cæca. The second divides into several long _mesenteric arteries_ supplying blood to all parts of the intestine and spleen. In the caudal region blood is taken up through the _caudal vein_ and carried forward to the kidneys. These strain out the impurities arising from waste of tissues, after which the blood is carried back to the sinus venosus through the cardinal vein. From the intestine it is gathered into the large _portal vein_ as in the toad. The portal vein carries blood to the liver, where nutriment may be stored up, and from thence it flows back to the sinus venosus through a very short thin-walled vessel, the _hepatic sinus_. The _kidneys_, more or less united in one mass, lie in the posterior part of the body-cavity along the dorsal wall. Note running from each side of the kidney a _ureter_ which unites with its fellow and opens into a small _urinary bladder_ which discharges through a small opening immediately back of the anus. The reproductive organs lie below the swim-bladder near the posterior end of the body-cavity. If the fish are caught in the spring, the greater part of the body-cavity of the female is found to be filled with small eggs. When mature, these eggs are deposited by the mother fish in the gravel of the stream-bed where they are fertilized by the sperm-cells poured over them by the male and left floating in the water.