It contains several rows of valves, variable in number, but most numerous in bony Ganoidei. One row is especially prominent in Dipnoi, and in Protopterus and Lepidopterus the valves in this row coalesce into a ridge. It is opposed by another ridge of coalesced valves, and with these two ridges there coalesces also in both Protopterus and Lepidosiren a septum which cuts off the origins of the two anterior from the origins of the two posterior pairs of aortic arches. The septum is incomplete in Ceratodus. The effect of this arrangement, coupled with the peculiar structure of the heart, is to send into the two first pairs of arches in Ceratodus a current of mixed arterial and venous blood; of pure arterial in the other two Dipnoans (Boas).

A sub-branchial artery or ventral aorta springs from the conus, or from the ventricle in Teleostei, where its entrance is guarded by a pair of valves, remnants of the anterior row of the conus. The commencement of this vessel is dilated, and is termed bulbus aortae. It gives origin to the aortic arches, or branchial arteries, which are placed at some distance apart one from the other except in Dipnoi, where their roots are close together, as in Amphibia. There are five branchial arteries in Elasmobranchii, Holocephali, chondrostean Ganoidei and Lepidosteus; the first supplies the hyoidean, or opercular gill, the remainder the gills of the branchial arches. Hexanchus and Heptanchus have respectively one and two additional arches. The hyoidean vessel springs from the first branchial artery, not from the ventral aorta, in Polypterus and Protopterus, and carries venous blood; but it supplies no gill in the first-named Fish. The hyoidean vessel is an artery carrying arterial blood and supplying the pseudobranchia in Teleostei, and takes origin from the ventral end of the first branchial vein. In this order there are typically four branchial arteries. The branchial veins fall dorsally either into a median vessel or into a right and left epibranchial artery, which fuse into a median vessel.

This vessel in either case is the sub-vertebral aorta, which is continued to the tip of the tail as the caudal artery. The common carotids arise from the dorsal extremity of the hyoidean vein or, in Teleostei, artery (supra), which is connected dorsally to the subvertebral aorta or epibranchial artery. The two carotids are connected across the base of the skull (except in Dipnoi) by a vessel, thus making a circulus cephalicus. The internal carotids arise from this connecting vessel. The venous blood returns to the sinus venosus by a right and left ductus Cuvieri, formed by the fusion of the jugular and subclavian veins with the posterior cardinal veins (p. 352). The right ductus in Lepidosteus, the two ductus in Polypterus, enter the auricle, not the sinus. There is a hepatic-portal and a renal-portal system, the latter supplied by the branches of the caudal vein. The hepatic veins enter the sinus venosus independently, and there is no vena cava inferior. The lymphatic system consists of vessels lodged in the myocommata and other fibrous septa, and of spaces surrounding the chief vessels, and a right and left or a single sub-vertebral canal. Masses of adenoid or lymphatic tissue imbed the kidneys and the testes in Dipnoi, and are connected across the median line by several bands.

And in some Ganoids (Acipenser, Lepidosteus) and many Teleostei the anterior part of the kidney is converted into a vascular adenoid tissue. Lymphatic hearts, single in the Eel (Anguilla) or double in Silurus, open into the caudal vein. The coelome opens externally by abdominal pores in most Elasmobranchii (except four genera of Scyllidae, Cestracion, Notidanidae, and Rhinidae), in Holocephali, Ganoidei, some phy-sostomous Teleostei (female Salmonidae, Mormyrus, Muraenoidei), and the Dipnoi. The two pores lie close to the anus in Holocephali, Ganoidei, Salmonidae, and Mormyrus, close behind the cloaca in Ceratodus, at its margin, or just within it, in Elasmobranchii. When double in Protopterus, they open into the cloaca behind the rectum; when single, externally and in front of it, and either within or without the limits of the sphincter muscle. There is a single pore in Muraenoidei, which opens into the ureter close to its external aperture.

The thyroid gland is a large mass in Elasmobranchii, lying at the anterior end of the ventral aorta: in Teleostei it is represented by masses of reddish follicles lying ventrally to the same vessel, and scattered for a greater or less distance along the branchial arteries. The thymus is paired: each gland lies at the dorsal extremities of the branchial sacs in Elasmobranchii: at the dorsal ends of the last pair of branchial arches close beneath the mucous membrane of the branchial cavity in Ganoidei and Teleostei. In the latter it is largest in half-grown individuals, and atrophies in the adult. A spleen is always present.

The pronephros seen in the embryo Acipenser, Lepidosteus, and Teleostean appears to atrophy completely. The mesonephros forms the permanent kidney in Ganoidei, Teleostei, and Dipnoi, but in Elasmobranchii its hinder portion becomes specially enlarged, quite or nearly independent of the fore-part, and acquiring ducts of its own, represents the metanephros of higher Vertebrata. The anterior portion of the mesonephros is in this case converted into epididymis in the male, parovarium in the female. The size and form of the mesonephros are very variable. It is especially large in chondrostean Ganoidei, and it extends in many Teleostei far into the caudal canal. The two glands fuse anteriorly and posteriorly in chondrostean Ganoidei; and in some Teleosteieven for their whole length. In bony Ganoidei and Dipnoi they are short, and restricted to the posterior part of the coelome. In all cases they lie subvertebrally. The metanephros of Elasmobranchii is more or less lobulated. The duct lies sometimes in the substance of the gland in Teleostei, at its outer margin in Ganoidei, on the ventral surface in Dipnoi, and the several ducts (rarely a single duct) in Elasmobranchii proceed from its inner border.