Fish are very rarely edentulous, e. g. Acipenser, some Lophobranchii, but the former has larval teeth afterwards lost. Protopterus among Dipnoi, has two conical, Ceratodus two elongate vomerine teeth, whilst the palato-pterygoid bone and the splenial bear peculiar ridged plates. The palato-pterygoid cartilage and the mandible are dentigerous in Elasmobranchii and Holocephali. In bony Ganoidei and Teleostei the praemaxillae, vomers, parasphenoid, dentary, and branchial arches must be added to the list. The maxilla carries teeth in bony Ganoidei, but only in a few Teleostei, e.g. Salmonidae. The teeth are composed of dentine, which may obliterate the pulp cavity, and is coated or tipped with structureless enamel. The dentine in Lepidosteus is folded, as in many Stegocephali (Amphibia); and the plates of Myliobatis (Batoidei) are composed of hexagonal denticles set side by side, as in Orycteropus among Edentate Mammalia. The form of the teeth varies much: serrated or pointed plates in many Sharks: flat plates in Cestracion and some Rays (Myliobatis); very various but usually conical in Ganoidei and Teleostei. They are attached or implanted in a fibrous membrane, which moves forwards over the edge of the jaw in Elasmobranchii; by an elastic ligament which allows the tooth to bend backwards in some Gadidae, Lophius, the vomerine and palatal teeth of Esox, among Teleostei; in sockets, e.g. in Sphyraena (Barracuda Pike), and a few other Teleostei, to which they may be anchylosed, as in Lepidosteus; or by special 'bone of attachment' the most common mode.
The formation of new teeth is in most instances perpetual, and in Teleostei the new-enamel germs are derived from the rete mucosum, and not, as usual, from pre-existing germs. Vertical replacement is found in the pharyngeal teeth of Pharyngognathi.
Fish have no oral salivary glands. The tongue is a thickening of the mucous membrane covering the basihyal, with free edges and apex in Elasmobranchii and Polypterus. In the latter it contains muscular tissue, and special lingual muscles are said to be present in Dipnoi. The mouth is a curved or straight transverse aperture, ventral in Elasmobranchii, terminal in other Fish. The pharynx is perforated laterally by the gill-slits. The digestive tract takes a nearly straight course in Holocephali, a few Teleostei, and Dipnoi. The oesophagus is scarcely marked off from the stomach. It has sometimes pointed processes directed towards the stomach (some Elasmobranchii and Teleostei); large and branched in the basking Shark (Selache). Ciliated epithelium occurs in it in Elasmobranchii and Ganoidei. The stomach is a dilatation followed by a pyloric constriction in some Teleostei, e. g. Gobius, Protopterus; a siphonal bend, or a caecal pouch with a lateral pylorus in Amia, Polypterus, and very many Teleostei. In some of the latter the gastric glands have an epithelium unlike that of the stomach. The intestine is usually dilated at the pylorus, and in Elasmobranchii the dilatation is known as Bursa Entiana. It gives off in this region, in Ganoidei and most Teleostei (except e. g.
Siluroidei, Esox, Plectognathi) caecal processes or appendices pyloricae, variable in number; arranged in a whorl or a linear series (Salmonidae); opening each by a separate aperture or united into bundles with a common duct; free or bound together by connective tissue into a solid mass in Acipenser and a few Teleostei, e. g. Tunny (Thynnus). Their epithelium may be ciliated (cf. p. 87). The remainder of the intestine is often straight, but in Teleostei may be somewhat convoluted. It contains in Elasmobranchii, Holocephali, Ganoidei, and Dipnoi a spiral ridge or valve, very well-developed in the two orders first named; but reduced in chondrostean Ganoidei, Lepidosteus, and Amia. In some Elasmobranchii, e.g. Zygaena, Carcharias, it has the form of a lamella rolled upon itself. A rudiment of it is said to exist in the Clupeid Chirocentrus (Teleostei). The rectum is clearly marked off from the intestine by being dilated in Ganoidei. There are no intestinal glands, but goblet mucous cells occur in the epithelium. The rectum is lined by the same epithelium as the intestine, except in Elasmobranchii, where it is squamous.
Ciliated epithelium is found on the edge of the spiral valve in Elasmobranchii ox here and there in some Teleostei, e.g. John Dory (Zeus faber). Peculiarities are the presence of a rectal gland in Elasmobranchii; striation of the muscular coats of the digestive tract in the Tench (Tinca); the presence of capillaries between the epithelium cells of the intestine in the air-swallowing Loach (Cobitis fossilis). The dorsal mesentery often becomes largely fenestrated, sometimes almost absorbed. A nearly complete ventral mesentery (p. 357) is found in the Dipnoi, a few other Fish (Muraenoids), and a posterior rudiment in Lepidosteus. The rectum ends in a cloaca in Elasmobranchii and Dipnoi common to it and the urogenital ducts. The anus in other Fish lies in front of the urogenital apertures.
The liver is unilobar in Lepidosteus and a few Teleostei, bi- or tri-lobed in other Fish. The two lobes are connected by peritoneum only in some Rays, e.g. Torpedo. They are of huge size, reaching to the cloaca in some Sharks. A gall-bladder is rarely absent, and is either imbedded in the liver or lies between its lobes; it varies much in form and size, e. g. it may reach to the anus, as in Scomberidae (Mackerels). There are several bile-ducts, but they usually unite into a single duct which opens near the pylorus. A uni- or bi-lobed pancreas is present in Elasmobranchii: it is large and lobed in Acipenser, small in Lepidosteus (? rudimentary), and a few Teleostei, e. g. Esox, but in the last named order it is generally either a diffused gland or absent altogether.
The external gill-clefts are exposed in all Elasmobranchii, and the first (post-mandibular) or spiracle is always so when present. They are protected in other orders by an opercular fold from the hyoid arch, in which opercular bones and sometimes branchiostegal rays (p. 93) are formed except in Holocephali. The spiracle is found in Elasmobranchii, in Acipenser and Spatularia among chondrostean, and Polypterus among bony Ganoidei. Its gill, if retained, derives arterial blood from the hyoidean or opercular gill. The post-hyoidean gill pouches of the embryo retain the character of pouches in Elasmobranchii, where they are five in number, except in Hexanchus and Heptanchus, with six and seven respectively; and in Holocephali, where they are four. The septa, which are continued outwards from the branchial arches, atrophy more or less completely in all other Fish, and thus leave the gill-filaments projecting freely, at least at their apices. The respiratory tissue has the form of radiating folds when the septa are complete; of filaments supported by an axial cartilage when they are atrophied; longest about the centre of the arch or septum. The first set is borne upon the hyoid arch, or the operculum, when present: hence hyoidean or opercular gill.