of the ventral surface. With two African Ganoids, Polypterus and Calamoichthys (as also stated by Müller, and verified by the writer as to the latter genus) the sac is double, and communicates with the ventral side in the median line; but it is slightly cellular, as in Menobranchus.
Fig. 7.—Diagrams representing the Connection between the Air-bladder or Lung and the Alimentary Canal in Certain Vertebrates.
Al the alimentary canal, A, the air-bladder. AD. the air-duct.
The figures at the right show the alimentary canal and air-bladder from the left side; those at the left represent cross-sections more or less foreshortened in some cases.
A A′ represent the simple condition-connections of the air-bladder in the sturgeons (Acipenser) and in most Teleosts where the air duct remains open. B B′ represent the arrangement in Amia and Lepidosteus, where the duct opens upon the dorsal side of the throat, but the bladder is more or less cellular. The hinder end of the bladder is left open to indicate its great length in Lepidosteus. In C C′ is shown the arrangement in Erythrinus. The bladder is still upon the dorsal side, but the front part is separated from the hinder two-thirds by a constriction, and the long duct passes forward from just behind the constriction to enter the left side of the throat. There are fibrous partitions in part of the bladder, but I do not know that they are vascular. The condition in Ceratodus is shown at G; the bladder is single but vascular, and the duct opens on the ventral side, but not in the middle line.
In the remaining figures the air-duct opens on the lower or ventral surface of the throat, and the air-bladder is in two parts, which unite at the duct, but separate backward and lie upon the sides of the stomach, or even to some extent upon its dorsal surface next the back-bone. In the side-views only the left sac is seen; in the cross-sections the whole is fore-shortened so as to bring it into one plane. In Polypterus and Calamoichthys the inner surface of the sacs is nearly smooth, but in Lepidosiren, as in the salamanders, it is more or less folded and vascular, and is also connected with the heart by special vessels. In the reptiles, birds, and mammals, the duct or trachea soon divides into the two bronchial tubes.
Finally, in the "mudfishes" of Africa and South America (Protopterus and Lepidosiren) the duct is ventral, and the air-bladder is a double and lung-like sac with stiff walls.
This series seems to connect the air-bladder of the fishes with the lungs of the true aërial vertebrates, and to remove the objection
