It is found in Elasmobranchii, Holocephali, Acipenser, Scaphirhynchus, and Lepidosteus, among Ganoidei, where it receives the venous blood of the first branchial artery or a branch of that vein in Protopterus and Lepidosiren among Dipnoi1: in many Teleostei, where it is known as pseudobranchia (a term also applied to the spiracular gill of Ganoidei), and is supplied with blood from the hyoidean artery. This pseudobranchia has the form of either a set of short free filaments, e. g. Pleuronectidae, many Acanthopteri, or a vaso-ganglion beneath the mucous membrane, e.g. Esox, Cyprinus, Gadus. The remaining sets of gill-folds or filaments are borne upon the anterior and posterior aspects of the septa or branchial arches, the branchial vein and artery running at the base of each septum, or the outer aspect of the arch. The gills are typically biserial. The fourth branchial arch, however, bears only a single, the anterior, row of filaments in many Teleostei, e.g. Cyclop-terus, many Plectognathi, or no gill at all, e.g. Lophius; and in Malthaea among Pediculati (Acanthopteri) the third arch is uniserial, whilst the second only is gill-bearing in Amphipnous cuchia (Physostomi Apodes). The fifth arch has no gills except in Hexanchus and Heptanchus. Ceratodus among Dipnoi, has four biserial gills, and the septa are well developed; Protopterus has biserial gills on the third and fourth branchial arches, and only a single series on the fifth 'immigrants from the hind row of the fourth arch,' seeing that the blood-supply is derived from the fourth branchial artery (Boas). The gill-filaments of Protopterus are leaf-like. The Lophobranchii among Teleostei are peculiar in having tufted processes in the place of gill-filaments.

1 It is present in Ceratodus, but the source of its blood-supply is apparently unknown.

The gill-filaments are prolonged externally in embryo Elasmobranchii, in Acipenser, and many Teleostei, but are afterwards reduced. The young Polypterus has two pairs of external gills supplied from the hyoidean artery. Protopterus, among Dipnoi, has three pairs of external gills, supplied by the second, third, and fourth aortic arches, and attached close to the summit of the shoulder-girdle. Accessory respiratory organs are found in certain Teleostei. These take the form of (1) enlarged honey-combed superior pharnygeal bones in Anabas scandens (Climbing Perch) and its allies among Acanthopteri: (2) arborescent processes of the dorsal part of the branchial arches in Clarias, Heterobranchus, Heterotis, among Siluroidei: (3) a sac with vascular walls opening into the first gill-cleft in Amphipnous cuchia and Saccobranchus singio (Siluroidei). The above-named can exist out of water, but this is not the case with Lutodeira chanos, a Clupeid, which 'has an accessory gill developed in a curved caecal prolongation of the branchial cavity' (Huxley).

The outgrowth of the oesophagus, which forms the lungs of other Vertebrata, is absent in Elasmobranchii1, Holocephali, some Teleostei, e.g. Pleuronectidae, and various families and genera. Its aperture is ventral in Polypterus and Dipnoi, dorsal elsewhere; and in some Teleostei shifts backwards, even opening into the fundus of the stomach in Clupeidae. In this family the Herring has a duct leading from the hind end of the air-bladder, and opening on the left side of the sexual aperture. Protopterus has a divaricator muscle to the aperture, which leads into a vestibule in it, in Lebidosteus. Polvbterus, and Lepidosiren. There is scarcely any duct in Amia and chondrostean Ganoidei, but a long one, the ductus pneumaticus, is found in the Teleostean sub-order Physostomi, while it is absent in the remaining sub-orders, hence often grouped together as Physoklisti. The lumen of the duct is, however, sometimes closed. The sac itself is double in Polypterus, Protopterus, and Lepidosiren, single in other Fish: it lies between the alimentary tract below, the aorta and kidneys above: it is covered by peritoneum only on its ventral surface, and is sometimes protected in Cyprinoidei and Siluroidei by a partial or complete bony capsule.

Its inner surface is produced into ridges, regular in Lepidosteus and Ceratodus, irregular in Amia, Protopterus, Lepidosiren, and some Teleostei, or else it is smooth. It derives its supply of blood either from the aorta or caeliac artery in chondrostean Ganoidei, Lepidosteus, and Teleostei, or by two pulmonary arteries arising, as in Amphibia, from the fourth branchial veins (fourth aortic arches) in Polypterus, Amia, and Ceratodus, or by a single artery from the common aortic root of the left side, produced by the union of all the branchial veins, in Protopterus and Lepidosiren. Its blood is returned by a pulmonary vein into the left division of the sinus venosus in Dipnoi, into the systemic veins, either portal, hepatic, or cardinal in all other Fish. Its capillaries form radiating tufts in the Cyprinoidei: vaso-ganglia, i.e. bipolar retia, in many Teleosteans, e.g. Anguilla, Gadus, Perca. The function of the sac in most instances is almost entirely hydrostatic; hence the name air- or swimming-bladder: but when the air within can be renewed, as in Dipnoi especially, it must also act as a lung.

But in all Fish, when the branchiae are in full activity, it receives a supply of pure arterial blood.

1 Unless represented by a dorsal caecum in some Sharks.

In some Plectognathi, e.g. Diodon, an air sac opens into the ventral side of the pharynx. It is distensible, and when inflated, these Fish float on the surface, and are driven about by the currents.

The heart lies in a pericardial cavity, shut off from the rest of the coelome by an oblique or vertical fibrous septum. The two cavities communicate by a single canal, in Chimaera (Holocephali), Acipenser, and Spatularia, by a bifurcating canal in Elasmobranchii. The heart itself consists of a sinus venosus, auricle, ventricle, and a tubular prolongation of the latter containing several series of valves, the conus arteriosus, a structure which is aborted in all Teleostei except in the Clupeid Butirinus (p. 88). The sinus venosus is divided into a right systemic and a left pulmonary section in Dipnoi. It opens into the auricle by an elongated aperture guarded by two valve-like folds, or in bony Ganoidei and Dipnoi, by several eminences. The auricle is thin-walled, its muscles disposed in interlacing ridges, and its ventral wall coalesces with the conus in bony Ganoidei, less completely in Dipnoi. In Lepidosiren its cavity is divided by a fenestrated muscular septum; by a dorsal fibrous ridge in other Dipnoi. There are no auriculo-ventricular valves in this order: two valves with chordae tendineae in most other Fish: four in Amia, six in Polypterus and Lepidosteus. The walls of the ventricle are thick, and contain a lymphatic space in Teleostei and Ganoidei. Muscular ridges project into its cavity, especially in Dipnoi, and there is an incomplete septum in Lepidosiren. The conus is spirally twisted in Dipnoi, and at the same time slightly folded upon itself.