DIPNOANS, or lung fishes.  These are represented to-day by a few peculiar fishes and are distinguished by some high structures which ally them with amphibians.  An air sac with cellular spaces is connected with the gullet and serves as a rudimentary lung.  It corresponds with the swim bladder of most modern fishes, and appears to have had a common origin with it.  We may conceive that the primordial fishes not only had gills used in breathing air dissolved in water, but also developed a saclike pouch off the gullet.  This sac evolved along two distinct lines.  On the line of the ancestry of most modern fishes its duct was closed and it became the swim bladder used in flotation and balancing.  On another line of descent it was left open, air was swallowed into it, and it developed into the rudimentary lung of the dipnoans and into the more perfect lungs of the amphibians and other air-breathing vertebrates.

One of the ancient dipnoans is illustrated in Figure 300.  Some of the members of this order were, like the ostracoderms, cased in armor, but their higher rank is shown by their powerful jaws and by other structures.  Some of these armored fishes reached twenty-five feet in length and six feet across the head.  They were the tyrants of the Devonian seas.

GANOIDS.  These take their name from their enameled plates or scales of bone.  The few genera now surviving are the descendants of the tribes which swarmed in the Devonian seas.  A restoration of one of a leading order, the fringe-finned ganoids, is given in Figure 301.  The side fins, which correspond to the limbs of the higher vertebrates, are quite unlike those of most modern fishes.  Their rays, instead of radiating from a common base, fringe a central lobe which contains a cartilaginous axis.  The teeth of the Devonian ganoids show a complicated folded structure.

General characteristics of Devonian fishes.  The notochord is persistent.  The notochord is a continuous rod of cartilage, or gristle, which in the embryological growth of vertebrate animals supports the spinal nerve cord before the formation of the vertebrae.  In most modern fishes and in all higher vertebrates the notochord is gradually removed as the bodies of the vertebrae are formed about it; but in the Devonian fishes it persists through maturity and the vertebrae remain incomplete.

The skeleton is cartilaginous.  This also is an embryological characteristic.  In the Devonian fishes the vertebrae, as well as the other parts of the skeleton, have not ossified, or changed to bone, but remain in their primitive cartilaginous condition.

The tail fin is vertebrated. The backbone runs through the fin and is fringed above and below with its vertical rays.  In some fishes with vertebrated tail fins the fin is symmetric, and this seems to be the primitive type.  In others the tail fin is unsymmetric:  the backbone runs into the upper lobe, leaving the two lobes of unequal size.  In most modern fishes (the teleosts) the tail fin is not vertebrated:  the spinal column ends in a broad plate, to which the diverging fin rays are attached.