. 658 compressed, so that part of the contents of the posterior may be driven into the elastic anterior division, and vice verta. The posterior division being provided with the ductus pneumaticus does not require the elasticity of the anterior. Some Siluroids possess a peculiar apparatus for volun tarily exercising a pressure upon the air-bladder. From the first vertebra a process takes its origin on each side, expanding at its end into a large round plate; this is applied to the side of the air-bladder, and by pressing upon it expels the air through the duct; the small muscle moving the plate rises from the skull. The connexion of the air-bladder with the organ of hearing in some Physostomes has been described above, p. 653. In the modifications of the air-bladder hitherto mentioned, the chief and most general function is a mechanical one : this organ serves to regulate the speci fic gravity of the fish, to aid it in maintaining a par- fa s. ticular level in the water, in rising or sinking, in raising the front part of its body or depressing it, as occasion may require. Yet a secretion of gas from the blood into its cavity must take place; and if this be so, it is not at all impossible that an exchange of gases between the two kinds of blood is also effected by means of the extraordinary development of retia mira- bilia in many air-bladders. In all fishes the arteries of the air-bladder take their origin from the aorta or the system of the aorta, and its veins return either to the portal, the vertebral, or the hepatic veins; like the other organs of the abdominal cavity, it receives arterial blood and returns venous blood. Whilst the air-bladders of some Ganoids, anatomi cally as well as functionally, closely adhere to the Tele- osteous type, that of Amia is more cellular and lung- lilrp in i<- ini-prinr tVmn tka FIG. 49. Lung of CVmfo*<s, opened in its ior tlian the lower llulf to show its collul .; i . pouches- leleosteous air-bladder, and a ri R ht h M; b < left ll!llf; <% cellular D 7 , . , pouches; e, vena pulmonalis; f avte- FolypteruS approaches the fial blood-ves^l; oesophagus, opened Dipnoi, not only in having a to show s lotti3 / ) laterally divided air-bladder, but also in its pneumatic duct entering the ventral side of the oesophagus. The air- bladder of the Dipnoi possesses still further the anatomical characteristics of a lung and assumes its functions, though, as it coexists with gills, only periodically or in an auxiliary [CIRCULATION. manner. The ductus pneumaticus is a membranous bronchus, entering the ventral side of the oesophagus, and provided at its entrance with a glottis, In Ceratodus (fig. 19) the lung is still a single cavity, but with asymmetrical arrangement of its internal pouches; it has no pulmonary artery, but receives branches from the arteria cceliaca. Finally, in Lepidosiren and Protopterw the lung is com pletely divided into lateral halves, and by its cellular structure approaches most nearly that of a reptile; it is supplied with venous blood by a true pulmonary artery. ORGANS OF CIRCULATION. The blood-corpuscles of fishes, with one exception, are of an elliptic shape; this exception is Petromyzon, which possesses circular, flat, or slightly biconvex blood-corpuscles. They vary much in size; they are smallest in Teleosteaus and Oyclostomes, those of Acerina cernua measuring -g^y of an inch in their longitudinal, and ^V TF in their transverse diameter. So far as it is known at present the Salmonidce have the largest blood corpuscles among Teleosteans, those of the salmon measuring j-g 1 ^ by ^Vg- in., approaching those of the sturgeon. Those of the Chondropterygians are still larger; and finally, Lepidosiren has blood-corpuscles not much, smaller than those of Perennibranchiates, viz., "5T7 by -y-| T in. Branchiostoma is the only fish which does not possess red blood-corpuscles. Fishes, in common with the other vertebrates, are pro vided with a complete circulation for the body, with another equally complete for the organs of respiration, and with a particular abdominal circulation, terminating at the liver by means of the vena portad; but the peculiar char acteristic consists in this, that the branchial circulation alone is provided at its base with a muscular apparatus or heart, corresponding to the right half of the heart of mam malia and birds. The heart is situated between the branchial and abdo minal cavities, between the two halves of the scapulary arch, rarely farther behind, as in Symbranchidce. It is enclosed in a pericardium, generally separated entirely from the abdominal cavity by a diaphragma, which i?, in fact, the anterior portion of the peritoneum, strengthened by aponeurotic fibres. In some fishes, however, there is a communication between the pericardial and peritoneal sacs, viz., in the Chondropterygians and Acipenser, whilst in the Myxinoids the pericardial sac is merely a continuation of the peritoneum. Relatively to the size of the body, the heart is very small. It consists of three divisions : the atrium, with a large sinus venosus into which the veins enter; the ven tricle; and a conical hollow swelling at the beginning of the arterial system, the structure of which forms one of the most important characters used in the classification of fishes. In all Paleeichthyes (figs. 50 and 51) this swelling is still a division of the pulsating heart, being provided with a thick muscular stratum; it is not separated from the ventricle by two valves opposite to each other, but its interior is fitted with a plurality of valves, arranged in transverse series more or less numerous in the various groups of the subclass. Lepidosiren and Protopterus offer an example of a modification of this valvular arrangement, their valves being longitudinal, each valve in fact being formed by the confluence of several smaller ones situated behind one another. This Palaeichthyan type is called couus arteri- osus. In Cyclostomes and Teleosteans (fig. 52) the enlargement is a swelling of the artery, without muscular stratum and without contractility; with the exception of the Myxinoids, its walls are thick and fibrous with many trabecuke and
pouches, but it has no valves in its interior, and is separated
