Whilst the air-bladders of some Ganoids, anatomically as well as functionally, closely adhere to the Teleosteous type, that of Amia is more cellular and lung-like in its interior than the Teleosteous air-bladder, and Polypterus approaches the Dipnoi not only in having a laterally divided air-bladder but also in its pneumatic duct entering the ventral side of the œsophagus. The air-bladder of the Dipnoi possesses still more the anatomical characteristics of a lung and assumes its functions, though, as it co-exists with gills, only periodically or in an auxiliary manner. The ductus pneumaticus is a membranous bronchus, entering the ventral side of the œsophagus, and provided at its entrance with a glottis. In Ceratodus (Fig. [65]) the lung is still a single cavity, but with a symmetrical arrangement of its internal pouches; it has no pulmonal artery, but receives branches from the arteria cœliaca. Finally, in Lepidosiren and Protopterus the lung is completely 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.
Fig. 66.—Heart of Lepidosteus osseus.
I. External aspect. II. Conus arteriosus opened.
a, Atrium; b, Conus arteriosus; v, Ventricle; h, Branchial artery for 3d and 4th gill; k, for the second; l, for the first; m, branch for the opercular gill; d, Single valve at the base of the conus; e-g, Transverse rows of Ganoid valves.
CHAPTER X.
ORGANS OF CIRCULATION.
The Blood-corpuscles of fishes are, with one exception, 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 Teleosteans and Cyclostomes, those of Acerina cernua measuring 1/2461 of an inch in their longitudinal, and 1/3000 in their transverse diameter. As far as it is known at present the Salmonidæ have the largest blood-corpuscles among Teleosteans, those of the salmon measuring 1/1524 by 1/2460 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.—1/570 by 1/941 in. Branchiostoma is the only fish which does not possess red blood-corpuscles.
[See G. Gulliver, “Proc. Zool. Soc.,” 1862, p. 91; and 1870, p. 844; and 1872, p. 833.]
Fishes, in common with the other Vertebrates, are provided 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 portæ; but their peculiar character 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 Mammalia and Birds.
The Heart is situated between the branchial and abdominal cavities, between the two halves of the scapulary arch, rarely farther behind, as in Symbranchidæ. It is enclosed in a pericardium, generally entirely separated from the abdominal cavity by a diaphragma, which is, in fact, the anterior portion of the peritoneum, strengthened by aponeurotic fibres. However, in some fishes 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.