There is however an internal valve, a remnant perhaps of a spiral valve, which is stated to exist only in Chirocentrus (Clupeidae) among Teleostei. The folds of the mucous membrane vary much in character, and villi are rare. Peptic glands are well marked in the stomach of the Perch; they are sometimes absent. Ciliated epithelium occurs in the pyloric appendages. The epithelium of the intestine is columnar, and contains goblet-cells; and it throws out pseudopodial processes. The gall-bladder is never absent; its duct in the Perch opens near the base of the pyloric appendage, which crosses the oesophagus. The true pancreas is present in the Perch as a diffuse gland. The lobules are chiefly found along the veins of the pyloric appendages, and the left branch of the portal vein. The air-bladder is simple in shape, occupies the whole of the dorsal portion of the abdominal cavity, is firmly fixed laterally to the body-walls, and its ventral surface is covered by peritoneum. Its walls are thin, and there are many vaso-ganglia or retia mirabilia developed on its ventral surface anteriorly and internally.
There is no trace of the air-duct which connects the bladder with the digestive tract in the Teleostei Physostomi.
The inner surface of the operculum, or more strictly speaking, the posterior edge of the hyomandibular bone, carries a filamentous pseudo-branchia or opercular gill. In the adult Pike and many other Teleostei this structure is hidden under the mucous membrane. It is a functional hyoidean gill in the young, but in the adult it receives arterial blood from the hyoidean artery, and transmits it to the choroid gland of the eye. Each of the first four branchial arches carries gill-filaments arranged in two rows, i. e. they are biserial. These filaments are separated to their base as in all Teleostei. The artery and vein run on the convex side of the arch, the vein at a deeper level than the artery.
In the heart there is a sinus venosus constituted by the fusion of the right and left ductus Cuvieri. It receives the hepatic veins, and its aperture into the auricle has two thin valve-like folds. The walls of the auricle are thin, and muscles arranged more or less in a network. An anterior and posterior valve guard its entrance into the ventricle. This structure has a thick wall, which in most Teleostei and Ganodei is divisible into two muscular layers, an outer and an inner, separated by a space. This space is lymphatic, and its surfaces are covered by an endothelium. Two valves, a right and a left, guard the passage from the ventricle to the bulbus aortae. The conus arteriosus of the Elasmobranch and Ganoid has been absorbed into the ventricle of the Teleostean. A slight trace of it is found in some Teleostei, but in Butirinus (Clupeidae) alone are there two sets of valves. The persistent valves appear to be those of the distal row of the Ganoid and Elasmobranch. The ventral aorta gives off (1) the fourth and third branchial arteries, which have a common origin, and (2) the second and first arteries separately. The veins unite dorsally on each side into an epibranchial artery, and in the same manner as the arteries rise from the ventral aorta.
Each epibranchial gives off the common carotid anteriorly, and the two carotids unite by a cross vessel which passes above the parasphenoid. Posteriorly, the two epibranchials fuse to form the subvertebral aorta. By these unions a circulus cephalicus is formed from which the external and internal carotids arise anteriorly, and into which the hyoidean artery falls dorsally. This artery rises from the ventral end of the first branchial vein, and in its course it supplies the pseudobranchia. In the Perch a caeliaco-mesenteric artery, which supplies most of the abdominal viscera, springs from the right epibranchial before it fuses with its fellow. The blood from the abdominal viscera (stomach, pyloric appendages, spleen, intestine, air-bladder in part) flows into a hepatic portal system. The genital (spermatic or ovarian) vein enters the left ductus Cuvieri. The caudal vein when it enters the abdomen divides into a right and left branch. The former anastomoses with the right cardinal vein, and the latter, which is small, is also connected to the right cardinal. There is a renal-portal circulation. The left cardinal vein rises from the anterior part of the left kidney and is not directly connected to a branch of the caudal vein.
Both cardinal veins unite anteriorly, each with the corresponding jugular vein, forming the right and left superior cavae or ducts of Cuvier. A vein - the inferior jugular - lies dorsally to the heart. It is best developed on the left side, collects blood chiefly from the inferior part of the head, and falls into the left duct of Cuvier.
The thyroid gland is broken up into lobules, and lies ventrally to the ventral aorta, but some of its lobules are found distributed for a short distance along the roots of the branchial veins. They are red in colour. The thymus is paired, and each part lies behind and dorsally to the branchial arches, internally to the supra-clavicle so-called. The kidneys are placed on either side of the vertebral centra. They enlarge anteriorly and coalesce, and are perforated by the cardinal veins. This region of the kidneys in many Teleostei was found by Professor Balfour to consist of a vascular lymphatic tissue, and it does not, as is generally stated, appear to be a persistent head-kidney or pronephros. In the Perch the kidneys thin out posteriorly; in the Pike they thicken and extend beyond the abdominal cavity into the caudal region. There are two ureters which rise from the ventral surface of the glands. They are fine tubes which pass ventrally downward behind the posterior end of the air-bladder, unite, and at the place of union develope a large simple urinary bladder.
This structure, the form of which varies much in fish, does not correspond with the urinary bladder of higher animals, which is derived in development from the anterior (ventral) wall of the intestine, and either represents (Amphibia) or is a remnant (Sauropsida, Mammalia) of the allantois.