656 ICHTHYOLOGY [ORGANS OP A pancreas has been found hitherto in all Chondro- pterygians, in Acipenser, and in many Teleosteans. The spleen, which is substantially a lymphatic gland, may be mentioned here, as it is constantly situated in the immediate vicinity of the stomach, generally near its cardiac portion. With the exception of Branckiostoma, it is found in all fishes, and appears as a rounded or oblong organ of dark red colour. ORGANS OF KESPIRATION. Fishes breathe the air dissolved in water by means of gills or branchiae. The oxygen consumed by them is not that which forms the chemical constituent of the water, but that contained in the air which is dissolved in the water. Hence fishes transferred to water frurn which the air has been driven out by a high temperature, or in which the air absorbed by them is not replaced, are speedily suffocated. The absorption of oxygen by fishes is comparatively small; it has been calculated that a man consumes fifty thousand times more than is required by a tench. Some fishes, how ever, evidently require a much larger supply of oxygen than others : eels and carps, and other fishes of similar low vitality, can survive removal from their element for days, the small quantity of moisture retained in their gill- cavity being sufficient to sustain life, whilst other fishes, especially such as have very wide gill-openings, are immedi ately suffocated after being taken out of the water. In some fishes noted for their muscular activity, like the Scomhridce, the respiratory process is so energetic as to raise the temperature of their blood far beyond that of the medium in which they live. A few fishes, especially such as are periodically compelled to live in water thickened into mud by desiccation and vitiated by decomposing substances, breathe atmospheric air, and generally have special contrivances for this purpose. These are so much habituated to breathing air that many of them, even when brought into pure water of normal condition, are obliged to rise to the surface at frequent intervals to take in a quantity of air, and, if they are kept beneath the surface by means of a gauze net, they perish from suffocation. The special contrivances consist of additional respiratory organs, lodged in cavities either adjoining the gill-cavity or com municating with the ventral side of the oesophagus, or of the air-bladder which enters upon respiratory functions (Dipnoi, Lepidosteus, Amia). The water used by fishes for respiration is received by the mouth, driven to the gills by an action similar to that of swallowing, and expelled by the gill-openings, of which there may be one or several on each side behind the head, or rarely one only in the median line of the ventral surface. The gills or branchiae consist essentially of folds of the mucous membrane of the gill-cavity (laminae branchiales), in which the capillary vessels are distributed. In all fishes the gills are lodged in a cavity, but during the embryonic stage the Chondropterygians have the gill-laminae extended into long filaments projecting beyond the gill-cavity, and in a few young Ganoids external gills are superadded to the internal. In Branchiostoma the dilated pharynx is perforated by numerous clefts, supported by cartilaginous rods (fig. 30, 7t). The water passes between these clefts into the peritoneal cavity, and makes its exit by the porus abdominalis, situated considerably in advance of the vent. The water is pro pelled by cilia. In the Cyclostomes the gills of each side are lodged in a series of six or more antero-posteriorly compressed sacs, separated from each other by intervening septa. Each sac communicates by an inner duct with the oesophagus, the water being expelled by an outer duct. In Bdellostoma each outer duct has a separate opening, but in Myxine all the outer ducts pass outwards by one common gill-opening on each side. In the lampreys the ducts are short, the outer ones having separate openings (fig. 1). The inner ducts lead into a single diverticulum or bronchus, situated below the oesophagus, blind behind, and communicating in front with the pharynx, where it is provided with two valves by which the regurgitation of the water into the buccal cavity is prevented. The same type of branchial organs persists in Chondro pterygians, which possess five, rarely six or seven, flattened pouches with transversely plaited walls. The septa between them are supported by cartilaginous filaments rising from the hyoidean and branchial arches. Each pouch opens by a cleft outwards, and by an aperture into the pharynx, without intervening ducts. The anterior wall of the first pouch is supported by the hyoidean arch. Between the posterior wall of the first and the anterior of the second sac, and between the adjacent walls of the succeeding, a branchial arch with its two series of radiating cartilaginous filaments is interposed. Consequently the first and last pouches have one set of gill-laminoa only, viz., the first on its posterior and the last on its anterior wall. The so-called spiracles on the upper surface of the head of Chondroptery gians must be referred to in connexion with the respira tory organs. They are the external openings of a canal leading on each side into the pharynx, and situated generally close to and behind the orbit. They frequently possess valves or an irregularly indented margin, and are found in all species during the embryonic stage, but it is only in some that they remain persistent. The spiracles arr tfce remains of the first visceral cleft of the embryo, and in the foetal state long branchial filaments have been observed to protrude as from the other branchial clefts. The Holocepliali and Ganoidei show numerous deviations from the Chondropterygian type, all leading towards the Teleosteans. As a whole they take an intermediate posi tion between the preceding types and the Teleosteans, but they show a great variation among themselves, and have in common only the imperfect separation of the branchial sacs and the presence of a single outer bran chial aperture. In the Teleostei the gills with their supporting branchial arches lie in one undivided cavity ; more or less wide clefts between the arches lead from the pharynx to the gills, and a more or less wide opening gives exit to the water after it has washed the gills. The interbranchial clefts have sometimes nearly the same extent as the branchial arches ; sometimes they are reduced to small openings, the integu ments stretching from one arch to the other. Sometimes there is no cleft behind the fourth arch, in which case this arch has only a uniserial gill developed. The gill-opening likewise varies much in its extent, and when reduced to a foramen may be situated at any part of the posterior boundary of the head. In the Symbranchidae the gill- openings coalesce into a single narrow slit in the median line of the isthmus. In the majority of Teleosteans the integument of the concave side of the branchial arches develops a series of horny protuberances of various form, the so-called gill-rakers. These serve to catch any solid corpuscles or substances which would be carried into the gill-cavity with the water. In some fishes they are seti- form, and make a complete sieve, whilst in others they are merely rough tubercles, the action of which must be very incomplete if they have any function at all. The majority of Teleosteans possess four complete gills. The gills of the Teleosteans, as well as of the Ganoids, are sup ported by a series of solid cartilaginous or horny pointed rods,
arranged along the convex edges of the branchial arches. Arcbes
