URINARY ORGANS.] ICHTHYOLOGY 659 from the ventricle by two valves opposite to each other. This Teleostean type is called bulbus aortae. The sinus venosus sends the whole of the venous blood by a single orifice of its anterior convexity into the atrium ; two thin membranous valvules turned towards the atrium prevent the blood from re-entering the sinus. A pair of other valves between the atrium and ventricle have the same function. The walls of the ventricle are strong, and, Fig. 50. Fig. 51. FIG. 50 Heart of Lepidosteus osseus. I. External pspect. II. Conns arteriosus opened, a, atrium ; b, conus arteriosus ; v, ventricle ; h, branchial artery for third and fourth K H ; i", for the second ; I, for the first ; m, branch for the opercular gill ; d, single valve at the base of the conus ; e,f,g, transverse rows of Ganoid valves. FIG. 51. Heart of Ceratodus. a, atrium; 6, conus arteriosus; </, papillary valve within the conus ; ej,g, transverse rows of Ganoid valves ; h, i, anterior arcus aoitaj ; k, I, posterior arcus aorta: ; v, ventricle. internally, it is furnished with powerful fleshy trabeculse. The bulbus or conus arteriosus is prolonged into the branchial ar tery, which soon divides, sending off a branch to each branchial arch. On re turning from the respiratory organ the branchial veins assume the structure and functions of ar teries. Several branches are sent off to dif ferent portions of the head and to the heart, but the main trunks unite to form the great artery which carries the blood to the Viscera and to FIG. 52.-Bulbus aorta: of Xiphias yJadiits, opened, a all the parts of 8 ? ctlo " 1 through part of the wall of ventricle ; b, section ,1 through the bulbus ; c, Teleosteous valves of the ostium tne trunk and Mteriorom ; 0, accessory vulves, of rudimentary nature tail and which inconstant ; e, trabcculx eaiueai of the bulbus. therefore, represents the aorta of higher animals. The circulatory system of Branchiostoma and of the Dipnoi shows essential differences from that of other fishes. Branchiostoma is the only fish which does not possess a muscular heart, several cardinal portions of its vascular system being contractile. A great vein extends forwards along the caudal region below the notochord, and exhibits contractility in a forward direction ; it is bent anteriorly, passing into another tube-like pulsatory trunk, the branchial heart, which runs along the middle of the base of the pharynx, sending off branches on each side to the branchiae ; each of these branches has a small contractile dilatation (bulbillus) at its base. The two anterior branches pass directly into the aorta ; the others are branchial arteries, the blood of which returns by branchial veins emptying into the aorta. The blood of the intestinal veins is col lected in a contractile tube, the portal vein, below the in testine, and distributed over the rudimentary liver. Of all other fishes, the portal vein is contractile in Myxinoids only. In Dipnoi a rudimentary partition of the heart into a right and a left division has been observed ; this is limited to the ventricle in Ceratochis, but in Lepidosiren and Pro- topterus an incomplete septum has been observed in the atrium also. All Dipnoi have a pulmonal vein, which enters the atrium by a separate opening, provided with a valve. The pulmonal artery rises in Lepidosiren and Pro- topterus from an arch of the aorta, but in Ceratochis it is merely a subordinate branch, rising from the arteria cceliaca. URINARY ORGANS. In Branchiostoma no urinary organs have been found. In Myxinoids these organs are of very primitive struc ture ; they consist of a pair of ducts, extending from the urogenital poms through the abdominal cavity. Each duct sends off at regular intervals from its outer side a short wide branch (the nriniferous tube), which communi cates by a narrow opening with a blind sac. At the bottom of this sac there is a small vaso-ganglion (Malpighian corpuscle), by which the urine is secreted. In the lampreys the kidneys form a continuous gland- like body, with irregular detached small portions. The ureters coalesce before they terminate in the urogenital papilla. In Chondropterygians the kidneys occupy the posterior half or twortliirds of the back, of the abdominal cavity, outside the sac of the peritoneum (as in all fishes), which forms a firm tendinous horizontal septum. The kidneys of the two sides are never confluent, and generally show a convoluted or lobulated surface. The ureters are short : each is dilated into a pouch, and communicating with its fellow terminates by a single urethra (which also receives the vasa deferentia) behind the end of the rectum in the large common cloaca. In Ganoids the kidneys occupy a similar position as in Chondropterygians, but these fishes differ considerably with regard to the termination and the arrangement of the ends of the urogenital ducts. The kidneys of Teleosteans are likewise situated outside the peritoneal cavity, immediately below some part of the vertebral column, and vary exceedingly with regard to form and extent. Sometimes they reach from the skull to between the muscles of the tail, sometimes they are limited to the foremost part of the abdominal cavity (in advance of the diaphragm), but generally their extent corresponds to that of the abdominal portion of the vertebral column. The ureters terminate, either separate or united, in a urinary bladder, varying in shape, which opens by a short urethra behind the vent. The urinary opening may be separate from or confluent with that of the genital ducts, and is frequently placed on a more or less prominent papilla (papilla urogeni-
talis). If separate, the urinary opening is behind the
